The Many Methods For Supplementing Calcium And Alkalinity

Posted by on January 1, 2015 - zero

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By Randy Holmes-Farley

There is no aspect of reef aquarium chemistry more important than calcium and alkalinity. Many of my previous articles have described various aspects of these systems in detail. In reading those articles, aquarists will note one pervasive theme: that maintaining appropriate levels of each are very important.

Moreover, the easiest way to ensure that things do not go seriously wrong in adding these to the aquarium is to use additives that have balanced amounts of calcium and alkalinity. For purposes of this article, a balanced calcium and alkalinity additive is one that provides calcium and alkalinity in proportions that match that used by corals and other organisms to form calcium carbonate. Using this type of additive typically prevents overdosing (or under-dosing) of either of these two relative to the other. There are several advantages to using these combined methods, and those advantages as well as the methods themselves will be detailed below. It is hoped that this article will provide the information necessary for aquarists to choose an appropriate scheme for their specific needs. Such deciding factors as cost, complexity, impurities, and a variety of others things come into play in making a decision.

One can, of course, supplement calcium and alkalinity independently, using, for example, calcium chloride and sodium carbonate (or bicarbonate). Such methods are perfect for making corrections to calcium and alkalinity levels, but perhaps less than perfect for routine maintenance. While the regular use of such additives can work well, it frequently results in substantial imbalances between calcium and alkalinity. In a perfect world, with perfect test kits used perfectly and frequently by every aquarist, such additions would work out fine. All too often, however, they lead to imbalances, and sometimes to a roller coaster effect of high and low calcium and alkalinity as the aquarist struggles to get these fundamental parameters under control.

It is for this reason that I strongly encourage aquarists to select a balanced calcium and alkalinity supplementation scheme. There are, however, many such schemes to choose from. Towards the end of the article are two tables, one that outlines the cost aspects of each system, and one that summarizes some of the other differences. In the end, I don’t pick any one of these schemes as being best for all aquaria, although I do indicate what types of tanks each system works well for, and what types they don’t. For experienced aquarists, that will be all they need to make informed choices. For beginners, I’ve also included some guidelines at the end of the article that should help them integrate these various concerns and point them in the right direction for the type of tank that they are considering.

Aquarists should also not be averse to combining two or more of these schemes. In some cases there are substantial synergies that can be obtained from combining systems. Some of the more common combinations are also discussed below.

I will say that I do not believe that there are any other systems commonly used that are as good as these detailed here. So these choices should cover the systems that people ought to consider unless they have very peculiar situations (or something new is invented in the future).

I’ll also try to straighten out some misunderstandings that aquarists frequently have about them (e.g., issues around heavy metals, either added intentionally or present as impurities). I won’t, however, have space in this article to give exacting details about how each is to be used. In many cases, there are existing articles describing them.

The systems to be covered in this article are:

  • Limewater (aka kalkwasser), used in a reactor or not, and with vinegar or not
  • Calcium carbonate/carbon dioxide reactors (CaCO3/CO2 reactors)
  • Calcium carbonate used without a reactor
  • Calcium acetate
  • One part inorganic salt mixtures
  • Two/three-part liquid additive systems (DIY, commercial, Balling, etc.)
  • Water changes

For those wanting a discussion of my recommended levels of various parameters in a reef aquarium, including calcium and alkalinity, see:

Optimal Parameters for a Coral Reef Aquarium

And for further discussion of any of these issues, see my Reef Chemistry Forum at Reef2Reef here.


Limewater (also known by the German term kalkwasser) has been used very successfully by aquarists for decades, and it is the system that I have used exclusively on my aquarium for 19 years. It is comprised of an aqueous solution of calcium and hydroxide ions that can be made by dissolving either quicklime (calcium oxide, CaO) or lime (calcium hydroxide, Ca(OH)2) in fresh water. The only inherent difference between the two is that if you add a molecule of water to quicklime, you get lime, and that a significant amount of heat can be generated when that happens.

CaO + H2O → Ca(OH)2

Quicklime + Water → Lime

Consequently, dissolving quicklime can make the water quite warm, especially if an excess of solids are added. Most hobby companies sell solid calcium hydroxide as “kalkwasser” or some similar name, although the name technically only applies to the solution.

The calcium ions in the solution obviously supply calcium to the tank, and the hydroxide ions supply alkalinity. Hydroxide (OH) itself provides alkalinity (both by definition and as measured with an alkalinity test), but corals consume alkalinity as bicarbonate, not hydroxide. Fortunately, when limewater is used in a reef tank, it quickly combines with atmospheric and in- tank carbon dioxide (CO2) and bicarbonate (HCO3) to form bicarbonate and carbonate (CO3):

OH + CO2 → HCO3

OH + HCO3 → CO3 + H2O

Once in the aquarium at an acceptable pH, there is no concern that the alkalinity provided by limewater is any different than any other carbonate alkalinity supplement. The hydroxide immediately disappears into the bicarbonate/carbonate system. In other words, the amount of hydroxide present in aquarium water is really only a function of pH (regardless of what has been added), and at any pH below 9, it is an insignificant factor in alkalinity tests (much less than 0.1 dKH). Consequently, the fact that alkalinity is initially supplied as hydroxide is not to be viewed as problematic, except as it impacts pH (see below).

The fact that limewater is very basic (the pH is typically above 12) demands that the limewater be added slowly to an aquarium unless very small additions are made. The reason for slow addition is two-fold: to prevent the local pH in the area of the addition from rising too high (slow addition permits more rapid mixing with tank water to reduce the pH), and to prevent the overall tank pH from rising too high (slow addition allows the tank to pull in CO2 from the atmosphere during the slow addition, mitigating the pH rise). Some aquarists advocate rapid addition, and that is acceptable for additions that would add significantly less than 0.5 dKH of alkalinity to the tank, but an addition of 1.4 dKH (0.5 meq/L; the equivalent of adding 1.2% of the tank volume in saturated limewater or 14 grams of solid calcium hydroxide into a 100-gallon tank) drives the pH of the whole tank too high (up by about 0.6 pH units from where ever it started).

Consequently, limewater is most often added slowly, by dripping or slow pumping. Often it is added as the top off water, replacing most or all of the evaporated water. The pumps add cost and complexity to the system, especially if combined with a float valve or switch (I use the latter and a Reef Filler pump).

As mentioned, limewater has a very high pH. This high pH can have significant advantages with respect to impurities present in the lime. Phosphate and many heavy metals will precipitate, either as calcium salts, or as metal oxides and hydroxides. Copper, for example, may accumulate in some aquaria. Copper hydroxide is very insoluble in limewater because of all of the hydroxide present. From an aquarist’s perspective, there will simply be no copper in clear limewater assuming that it has been given a chance to settle out because copper hydroxide is so very insoluble, regardless of whether there is a copper impurity in the calcium hydroxide solid, or in the source water used. Some aquarists get colored residues in limewater systems, and these colors are coming from metal impurities that did not get into the tank.

Another advantage of limewater may be its ability to reduce the phosphate already in the tank water. While it may be as simple as precipitation of calcium phosphate where the high pH, high calcium limewater meets the aquarium water, the mechanism and extent of this effect in typical reef tanks has not been established.

Another important consideration for limewater is the upper limit to the amount that can be added to an aquarium. The solubility limit of calcium hydroxide in fresh water is about 2 level teaspoons per gallon. If an aquarist has a tank near the high end of calcium and alkalinity demand, then replacing all of the evaporated water with saturated limewater may not be adequate to replace the ongoing losses of calcium and alkalinity. There are a couple of tricks to get a little more from the limewater. These are adding fans to increase evaporation, and adding vinegar to increase the solubility of the lime in the limewater (45 mL of vinegar per gallon of limewater will allow three level teaspoons to dissolve instead of just two). Both of these systems have been successfully employed by many aquarists.

Additionally, the use of a small amount of one of the other balanced additive systems (especially the two/three-part additive systems) in conjunction with limewater is often used by aquarists give a little boost to tanks that need a small amount of extra calcium and alkalinity beyond what limewater can supply, without incurring significant capital costs. Likewise, they can be successfully combined with limewater during periods of low evaporation. Unlike some other supplementation schemes, tank salinity will not increase over time through the use of limewater.

The cost of a limewater system can range from very little to quite a lot. If one uses an inexpensive drip system ($20) and bulk sources of lime the cost can be quite low. Bulk calcium hydroxide available to hobbyists sells for less than $2 per pound (maybe much less in a group buy from a large distributor). The cost per thousand milliequivalents (meq) of alkalinity is on the order of $0.15. I realize that this number means nothing to most aquarists, but I’ll use it to permit cost comparisons of very different supplementation schemes, and at the end of the article, I’ll convert it to yearly costs for some typical tanks. Branded hobby and lab grades of calcium hydroxide will be more expensive. A pound of calcium hydroxide from a well-known hobby company costs about $7, or $0.57 per thousand meq of alkalinity.

Of course, dosing pumps can be several hundred dollars, a good float switch can be $50-100, and one needs to get a reservoir as well (often a plastic container like a trash can; I use 44-gallon Rubbermaid Brute trash cans). Depending on the setup, the limewater reservoir can be far from the tank; even in another room or on another floor of the home. A pump like a Reef Filler or Liter Meter pump can be used to send the limewater significant distances, freeing up space around the tank.

Some people use reactors to deliver limewater. These systems automate the delivery of limewater to the tank, and, of course, the costs rise. They consist of a chamber where fresh water enters, is mixed with solid lime, and the fluid limewater exits the system and travels to the tank. They do not permit any additional calcium or alkalinity to be delivered to a tank compared to other limewater delivery methods (assuming that both use saturated limewater), but many claim them to be less hassle than delivery from a still reservoir. Addition of limewater with the simplest drippers may require daily attention, while delivery from a large reservoir may require attention only once every 1-5 weeks, which is about the same as typical limewater reactors. All of the other comments about limewater apply equally well when used with a reactor, a dripper, or a slow pump from a still reservoir (except that the vinegar/limewater combination is technically difficult to use with a limewater reactor).

On the negative side, limewater does have some concerns that don’t apply to most other systems. One is the effect of overdosing. All calcium and alkalinity additives, if added in sufficient overdose, can case abiotic precipitation of calcium carbonate in the tank. Limewater, however, is especially prone to this effect for two reasons. If overdosed, the high pH of the limewater will rapidly convert much of the bicarbonate in the tank to carbonate, increasing the likelihood of precipitating calcium carbonate. Also, addition of solid lime particles can cause local extreme spikes in pH and calcium that nucleate precipitation of calcium carbonate. Consequently, a limewater overdose, and especially the dosing of lime solids, is by far the most frequent cause of “snowstorm” events where calcium carbonate precipitates all through the water column. In some cases, the tank can look like milk. The good news is that this event usually causes no lasting harm to tank inhabitants unless the amount overdosed is exceptionally large, but it is nearly always upsetting to the aquarist. I’ve had it happen numerous times without losing anything.

Another drawback to systems where the limewater dose is tied to evaporation is that the evaporation may change daily or seasonally. I’ve not found that to be problematic in my system, but others who are more concerned about maintaining a very specific alkalinity may have more trouble with this issue. Dosing limewater on a timed pump rather than to match evaporation may eliminate the concern, as long as it doesn’t exceed evaporation rates.

One final note on lime: The high pH of the liquid and the dust hazard of the solid are not to be treated lightly. Inhalation of the dust is to be avoided. Splashing of limewater onto skin is also to be avoided, and should be followed by extensive rinsing with tap water if it happens. Splashing of limewater into the eyes is especially to be avoided, and the use of safety goggles when using large amounts or in situations where exposure is likely is prudent. Extensive and immediate rinsing with tap water, followed by professional help would be advised in the case of eye exposure.

Calcium Carbonate/Carbon Dioxide Reactors

Calcium carbonate/carbon dioxide reactors work by removing water from the tank, adding carbon dioxide to reduce the pH to about pH 6.5, and then allowing the more acidic water to dissolve solid calcium carbonate media that is present in a mixing chamber. The water is then returned to the tank with its extra calcium and alkalinity (bicarbonate):

CaCO3 + H+ → Ca2+ + HCO3

Reef tanks employing such reactors typically run at a pH below that of natural seawater, with typical tank pH values of 7.7 to 8.1. The reason for the low pH is the constant delivery of low pH solution to the tank, adding both excess CO2 and bicarbonate. There is no way around this completely, but some reactors incorporate a second chamber, allowing the liquid to pass over additional calcium carbonate media, making better use of the carbon dioxide that is actually added. Aquaria then blow off this extra CO2 and the pH rises, but the effect is typically not complete, and the pH often stays below what would be the case if the same tank water were fully aerated (that is, equilibrated) with normal air.

The media used is important in these systems, with the aragonite form of calcium carbonate being more readily dissolved than the calcite form (although both work). Also, the nature of the impurities can be very important, as nearly all of the impurities will be dissolved and delivered to the tank. Some of these impurities may be desired by the aquarist (such as magnesium and strontium) and some may not be (such as phosphate or copper). Phosphate in reactor media has sometimes become a point of competition between commercial suppliers of media for such reactors, but I would advise aquarists to be skeptical of some of these claims.

One big advantage of these reactors is that they can be scaled to deliver any amount of calcium and alkalinity needed by any tank. For this reason, they are greatly favored by those who have tanks with a high demand for calcium and alkalinity. Because of the low pH that often results, many of these aquarists choose to dose limewater in conjunction with the reactor, not because the reactor cannot supply enough calcium and alkalinity, but purely to raise the pH in the tank itself. The synergy between limewater and CaCO3/CO2 reactors involves more than just pH. Limewater uses up CO2 and CaCO3/CO2 reactors deliver it to the tank. Together, they combine to keep CO2 (and consequently, pH) more in line with natural seawater.

Calcium carbonate/carbon dioxide reactors take up a substantial amount of space, since one needs a carbon dioxide cylinder, a reaction chamber, and a pump. Typically, these systems are used close to a tank, but they could be remote if appropriate water flows to and from the tank could be worked out. Once an aquarist has properly adjusted the reactor system, it requires minimal monitoring for a substantial period. Tank salinity will not increase over time using calcium carbonate/carbon dioxide reactors.

The likelihood of problems from overdosing using such a reactor is minimal. Since the pH is typically low, even substantially elevated calcium and alkalinity values may not cause a dramatic calcium carbonate precipitation event. More likely is just slow precipitation onto heaters and pump impellers. Accidental delivery of large amounts of CO2 to the tank is a concern, but that is a rare accident.

The initial costs of such reactor systems can be considerable, typically about $300-600 for the reactor itself, plus additional costs for the CO2 apparatus. Media costs vary, but a bit over $2 per pounds is typical. That puts the media cost at about $0.30 per thousand meq of alkalinity. DIY ground limestone can be used as media for a tiny fraction of this cost, if you can find it locally. The carbon dioxide cost also needs to be figured in, so that might push the total to about $0.40 per thousand meq of alkalinity.

The primary safety concern for these systems involves the carbon dioxide gas cylinder. Any high-pressure gas cylinder can be very dangerous if the cylinder head should become damaged. So be careful to not drop such cylinders least they become rockets.

Calcium Carbonate without a Reactor

In a previous article, I described in detail what one could do with calcium carbonate when not used in a reactor. In my opinion, the best use is to dissolve the calcium carbonate in fresh water, and use it as the top off water for the system. Other uses, such as adding particulate or milky products directly to the tank seem like poor practice to me (since particulate calcium carbonate likely won’t dissolve in a reef tank and may actually nucleate precipitation of additional calcium and magnesium carbonate from the water).

The big drawback to this method is that not much calcium carbonate will dissolve in fresh water, regardless of what form the material takes (including fine aragonite particles). One is limited to about 30-ppm calcium in such top off water, which is about 25 times less than is present in saturated limewater. Consequently, this system alone is only good for aquaria with very low calcium and alkalinity demand, though it can be used in conjunction with just about any other supplementation system (except limewater, which uses the same top off water).

If you use products like commercial aragonite play sand for this application, the cost can be very low (if you can find it). If you use hobby grade products, like Aragamight, the cost is more on the order of $12 per pound, or $4.4 per thousand meq of alkalinity. Tank salinity will not increase over time using calcium carbonate.

Another use of calcium carbonate is as the substrate in a reef tank. As organic molecules are degraded inside of the substrate, the pH can drop, and the calcium carbonate can dissolve just as it does inside of a CaCO3/CO2 reactor. This rate of dissolution ends up being slow, however, and typically cannot provide a tank with adequate amounts of calcium and alkalinity unless the demand is very low.

One-part balanced additive systems: Calcium Acetate

Calcium acetate is a product that has gotten relatively little publicity despite its apparent ease of use and the commercial availability to aquarists. In some ways it is similar to the combination of limewater and vinegar. When dissolved in water (fresh or salt), you have calcium ions and acetate ions. The acetate is rapidly metabolized by tank organisms to form bicarbonate, carbon dioxide, and water:

CH3CO2(acetate) + 2 O2 → HCO3 + CO2 + H2O

This equation suggests that pH of such tanks may stay near the low end of normal, because of the excess carbon dioxide, but the practical experience of people using calcium acetate suggests that this is not a big concern.

Calcium acetate will also facilitate the growth of bacteria and the reduction of nutrients in systems, similar to that with folks dosing vinegar or vodka for that purpose. It will also facilitate conversion of nitrate to nitrogen gas (N2) in anoxic regions of live sand and rock by providing the carbon source necessary for the process. The equation below shows the process that could take place:

5 CH3CO2(acetate) + 8 NO3→ 10 CO2 + 4 N2 + 13 OH + H2O

One of the sources of calcium acetate available to aquarists is Salifert’s All in One (a product that also contains some strontium, amino acids, and some trace elements). It is a liquid product that can be poured directly into a tank with no immediate concerns about pH. The current version of their commercial product is 250,000-mg/L calcium acetate, so it contains the equivalent of 3,160 meq/L of alkalinity. This product sells in the US for about $45/L. Consequently, it costs about $14 per thousand meq/L of alkalinity. That price makes it very expensive for an aquarium with a large demand for calcium and alkalinity, but the zero equipment cost (unless you automate it with a dosing pump) makes it attractive for small aquaria, especially nano-reef tanks.

I have no information on the purity of the material, or the exact nature of the “trace elements” in it. Everything in the bottle will be delivered to the tank. It poses no unusual safety concerns. The upper limit to how much calcium and alkalinity can be supplied to a tank in this fashion depends on two factors. If the metabolism of acetate is rapid and the dose is very high, oxygen might be depleted. If the conversion is slow then acetate can build up in the tank (not itself a significant concern except perhaps at very high levels where it might confound an alkalinity test). Habib Sekha of Salifert has indicated that using the doses recommended on the bottle will not lead to either of these issues being problematic.

Overdosing is not expected to be an unusual problem, but if one makes significant additions in this fashion, the alkalinity will take time to show up completely in the tank because the acetate takes time to be metabolized. Consequently, I’d wait a day after adding it to measure alkalinity. Calcium measurement won’t be similarly impacted. Tank salinity will not increase over time using calcium acetate.

One-part balanced additive systems: Salt Mixtures

Another type of balanced one part additive is comprised of a simple dry mixture of sodium bicarbonate (or carbonate) and calcium chloride. Just as with the two-part additives described below, this type of system can be further formulated to have a natural seawater residue after removal of calcium carbonate. Tropic Marin’s Biocalcium seems to fall into this category, though its written descriptions are notoriously difficult to interpret. It costs about $18 for 510 grams (estimated to contain about 1800 meq of alkalinity), so that puts the cost at about $9.70 per thousand meq of alkalinity. It claims to add 70 trace elements to the tank, along with the calcium and alkalinity, but doesn’t specify amounts for any of them.

You cannot mix this type of additive in water prior to adding it to a tank. If you do, the calcium will react with the carbonate present to form insoluble calcium carbonate. Consequently, the directions advise adding it directly to the tank. If you do, be sure to add it in a high flow area away from corals (like a sump), as the solids are reported to irritate corals if they land on them.

If you use a product like this, be sure to keep it as dry as possible, even to the extent of keeping it in a sealed container to keep out atmospheric moisture. If moisture enters the mixture, it may allow the formation of undesirable calcium carbonate.

Continual use of products like this will increase the salinity in the tank. The rise in salinity over time can be roughly calculated, though not knowing exactly what is in it makes the calculation only a ballpark figure. For every 1000 meq of alkalinity added in this fashion these products will deliver on the order of 60 grams of other ions to the tank. In a tank with a low calcification demand (defined below to be 18.3 thousand meq of alkalinity per year in a 100-gallon tank (0.4 dKH/day)) this effect will raise the salinity by 3 ppt per year (compared to a normal salinity of S =35). In a high demand tank (defined below to be 219 thousand meq of alkalinity per year in a 100-gallon tank (4.4 dKH/day), the salinity will rise by 35 ppt in a year, or approximately doubling the salinity. Consequently, the salinity should be monitored closely in using this type of additive, especially in a tank with high calcification rates.

Two-part Balanced Additive Systems

There are now a plethora of two-part balanced systems for supplementing calcium and alkalinity, as well as DIY recipes that I have published and for which suppliers sell quality DIY ingredients. These are always liquid additives that you add equally to tanks to supplement both calcium and alkalinity. In the DIY version, magnesium is added to the aquarium as a third solution, although it need not be added especially frequently. The rational for this type of product is that the bicarbonate and carbonate that one might like to dose to supplement alkalinity are not readily compatible with the calcium that is also needed. So one portion contains calcium and the other contains the alkalinity. When a DIY is used, the magnesium sulfate in it is not compatible with either part, so it needs its own solution.

In the simplest form, such a system would be provided by any calcium salt at one concentration in one bottle, and a carbonate alkalinity supplement in the other bottle. Within that constraint, manufacturers have a fair amount of room to play. Typically these additives claim go a step further. When the calcium and alkalinity are taken out of the picture, as they will be by calcification in the tank, then the ions that remain are often described as having the same ratios of ions as natural seawater. Assuming that this is true, then the “residue” is simply more salt for the aquarium. Over long periods of time the salinity will build up due to this process (an effect that is quantified below), but there will be no significant buildup of specific ions in the tank.

In order to accomplish this, manufacturers could use a variety of calcium salts in the calcium portion, for example. They could use calcium chloride, calcium sulfate, calcium bromide, and a variety of other similar salts. They could also put magnesium and strontium in this portion as they would not be compatible with the alkalinity component.

The alkalinity portion of these systems is more complicated. As has been shown in other parts of this article, alkalinity can be provided as bicarbonate, carbonate, or hydroxide. I don’t know of any commercial supplements that use hydroxide for a two part system, but the commercial ones do use bicarbonate, carbonate, and mixtures thereof. Consequently the pH varies substantially between brands, and the various brands of these products should not be thought of as identical for this reason, if no other. In order to attain the natural seawater residue, the alkalinity portion could contain sodium bicarbonate or carbonate, potassium bicarbonate or carbonate, lithium bicarbonate or carbonate, etc.

I’ve not seen any independent test of whether these actually produce a residue equivalent to natural seawater, but I’ve seen no particular reason to doubt it, at least for the major ions. When it comes to the trace elements that might concern some reef keepers, it seems unlikely that these products will be any less prone to having uncontrolled levels of trace compounds like copper than are commercial salt mixes, or any other supplement of calcium and alkalinity, but that remains to be determined (at least as far as I know).

One issue that has confused some reef keepers, however, is the presence of trace elements. Assuming that these products are actually formulated with every ion such that a true natural seawater residue remained (let’s call this the “ideal” product), then it will necessarily contain such ions as copper. Since copper is elevated in some reef tanks, and is toxic to many invertebrates, reef keepers have wrongly criticized this method as adding more copper. That’s actually not what would happen. Since these products leave a natural seawater residue, and since copper may be elevated in concentration in many reef tanks relative to seawater, then using these “ideal” products will actually LOWER copper levels because when the increase in salinity is corrected, the copper will drop.

For example:

You have copper in your aquarium at 4 ppb and salinity of S=35.

You add a two part additive that over the course of a month raises salinity to S=36, and raises copper to 4.02 ppb.

Then you correct the salinity back to S=35 by diluting everything in the tank with fresh water, and you get a final copper concentration of 3.9 ppb.

Does this happen in real products and not “ideal” products? I have no idea. But the statement by manufacturers that it contains all ions in natural ratios, including copper, should not be viewed as a concern that it is exacerbating a heavy metal problem.

The rise in salinity of these products over time can be very roughly calculated, though there are several reasons why this calculation is only an estimate. For every 1000 meq of alkalinity added in this fashion (and the matching amount of calcium) these products will deliver on the order of 60 grams of other ions to the tank. In a tank with a low calcification demand (defined later to be 18.3 thousand meq of alkalinity per year in a 100 gallon tank (0.4 dKH/day)) this effect will raise the salinity by 3 ppt per year (compared to a normal salinity of S ~35). In a high demand tank (defined later to be 219 thousand meq of alkalinity per year in a 100 gallon tank (4.4 dKH/day)), the salinity will rise by 35 ppt in a year, or approximately doubling the salinity. Consequently, the salinity should be monitored closely in using these types of additives, especially in a tank with high calcification rates.

Many people have begun to use dosing pumps to deliver these sorts of additives more uniformly across a day/night period with less work by the aquarist. Such pumps can be obtained starting under $100 for each part dosed this way. There is no need to dose the magnesium part this way, since very little is actually required and once a week is plenty often enough.

The costs of these systems vary a bit. The original B-ionic from ESV costs about $34 for 1 gallon of both parts (10,600 meq of alkalinity), or about $3.20 per thousand meq of alkalinity. It has a pH raising effect, similar to my DIY Recipe 1. The B-ionic Bicarbonate version is more expensive, and is necessarily more dilute than is the original because sodium bicarbonate is much less soluble than is sodium carbonate. If your tank pH gets too high using one of them (such as the original B-ionic), then it is reasonable to switch to one that has a smaller pH raising effect (like the bicarbonate B-ionic or my DIY Recipe 2 using baking soda).

The DIY recipes can be far less expensive, depending on what grade of ingredients you use. Buying ingredients from a place such as Bulk Reef Supply will cost roughly $10 per gallon (total cost of all parts, so 1 gallon calcium, 1 gallon alkalinity, and a few cups of magnesium additive), or about $1.40 per thousand meq of alkalinity.
Balling Method

The balling method seems to be similar to the two/three part methods that are more common in the United States. There are different variations used by different people, and some (Fauna Marin) have three parts just like the DIY recipes, with a calcium part, an alkalinity part, and a magnesium part. One distinguishing feature of these is that they explicitly add a trace element mix to some of them (two additives into the calcium part, one into the alkalinity part, none to the magnesium part).

The ingredients and amounts in these are proprietary and cannot be easily compared to other methods, but one should not assume that they are not present in commercial and even the DIY two/three part systems, either as intentional additives already in the commercial ones, or present as impurities in the main ingredients of the DIY.

It is not trivial to compare costs of the Balling method by Fauna Marin since it has many parts, but it seems that the cost is about the same as commercial two part systems, or $3.60 per thousand meq of alkalinity ($10 per gallon of the alkalinity part, $11.3 per gallon of the calcium part, and $4.75 for the magnesium equivalent amount).

Water Changes

The one thing going for water changes is that it is hard to screw them up chemically (aside from salinity, pH and temperature). The bad thing is that it is impossible to replace more than a small amount of lost calcium and alkalinity to an aquarium in this way. Some salt mixes are available with higher than natural seawater levels of calcium and alkalinity, so it may suffice for the very lowest demand aquaria, but it cannot keep up with alkalinity in a tank with rapidly growing coralline algae and/or hard corals unless the amount changed is on the order of 20-50% daily. Calcium is somewhat easier to maintain with water changes, since some salt mixes have quite excessive calcium levels (500+ ppm), but even that is not enough for most aquaria.

Cost Comparison

Each of the sections above has provided a cost estimate of using that system, though in some cases there are a variety of different options to choose from that can significantly impact cost. The table below is intended to be a very rough guide to the initial and yearly cost of each of these systems for three types of 100-gallon aquaria:

Tanks with a light calcification load, defined as 0.4 dKH/day (0.13 meq/L/day). This is the equivalent to the daily replacement of 0.3% of the tank volume with saturated limewater. This works out to 18,300 meq of alkalinity per year.

Tanks with a medium calcification load, defined as 1.1 dKH/day (0.4 meq/L/day). This is the equivalent to the daily replacement of 1% of the tank volume with saturated limewater. This works out to 55,000 meq of alkalinity per year.

Tanks with a heavy calcification load, defined as 4.5 dKH per day (1.6 meq/L/day). This is the equivalent of the daily replacement of 4% of the tank volume with saturated limewater (which usually cannot be attained). This works out to 219,000 meq of alkalinity per year.

Of course, smaller tanks will require less supplementation, and larger tanks will require more, and you can just scale the estimate to your tank based on its volume and your estimate of how much calcification is expected. Note also that some very high calcium and alkalinity demand tanks may be higher than the “high” demand tank.

As a general rule, a very small tank will probably be most economically served by a system with lowest set up costs (i.e., not a reactor of any kind), while for a larger amount of calcium and alkalinity, limewater and CaCO3/CO2 systems are likely to be the least expensive.

Table 1. Costs associated with various calcium and alkalinity supplementation schemes.
Yearly Cost for a 100 Gallon Tank ($)
Method Startup Costs ($) Light Load Medium Load Heavy Load
Limewater(Bulk Lime) 20-250+ 2.75 8.25 33.00 (not always possible)
Limewater(Aquarium Lime) 20-250+ 10.40 31.30 124.00 (not always possible)
CaCO3/CO2 reactor 350-650 7.32 22.00 87.60
CaCO3w/o reactor 0 (not possible) (not possible) (not possible)
Two-part systems (original B-ionic) 0-300 60.00 180.00 700.00
Two/three-part systems (DIY) 0-300 25.60 77.00 307.00
Three Part Balling Lite 0-300 66.00 198.00 788.00
One-part Calcium acetate 0-200 256.00 770.00 3066.00
One-part Salt Mixtures 0 178 534.00 2124.00


Summary of Properties

Table 2 is a summary of the properties of the various schemes that have been discussed throughout the article. Depending on the nature of the reef tank itself, some of these attributes may be more or less important, and it is up to each individual aquarist to decide what best fits their needs. In every instance, the entries in this table represent my opinions about things that are described in more detail in the text. Other aquarists may disagree about assessments of how complex or risky something actually is, however.

Table 2. Summary of attributes associated with various calcium and alkalinity supplementation schemes.
Method Limits to Addition Amount? Technical Complexity Daily Work by Aquarist? Overdose Risks Other comments
Limewater Moderate, based on evaporation medium Yes(no with equipment) medium Low metals
Reduced Phosphate
High pH
CaCO3/CO2reactor none high no low Delivers metals and strontium / magnesium
Low pH
CaCO3 w/o reactor Severe limits low Yes(no with equipment) none
Two/three-part systems(commercial or DIY or Balling) none low Yes(no with equipment) Low to medium Salinity Monitoring
Medium to High pH
One-part Calcium acetate None to moderate low Yes(no with equipment) low Carbon source
Medium pH
One-part Salt Mixtures none low yes low Must dose powder directly to tank
Salinity Monitoring
Medium pH

Summary Guidelines for Beginners

Less experiences aquarists may have some difficulty in deciding which of these various attributes are most important for their situation. In this section, I provide some guidelines in selecting a balanced calcium and alkalinity supplement for certain types of tanks. Much of what is detailed below is opinion, and other aquarists may have different opinions.

Very Small Reef Tanks

A very small tank (say, less than 10-20 gallons, especially those without a sump) will likely be served best by a system that does not involve the expenses, complications, and space requirements that come with reactors. Unless the calcification demand is very high, the costs associated with any of the simpler additives (the two-part systems, Balling; Salifert’s All in One, Tropic Marin’s Biocalcium) will probably not be prohibitive, and their ease of use makes them prime candidates. Without a sump, Biocalcium may be harder to add without solids getting onto organisms, so one of the other types may be a better choice. Simple drip limewater is also a less expensive possibility for these types of systems, but is best used when a sump is available.

Fish Only or Fish Only + Live Rock Tanks

These systems have smaller demands for calcium and alkalinity, though rapid coralline algae growth on live rock can itself provide a significant demand. Since the demands are lower than typical reef tanks, the simpler systems (the two-part systems, Salifert’s All in One, Tropic Marin’s Biocalcium) may be the best choices, but limewater is also a good choice for this type of system.

Large Reef Tanks

A large tank (say, more than 100 gallons) will likely be served best by a system that can deliver calcium and alkalinity at a reasonable unit price. Limewater, calcium carbonate/carbon dioxide reactors, and DIY two part systems are probably the best choices, with limewater not sufficing for the higher calcium demand tanks due to its limitation based on evaporation rates. If you are handy, you can put the system together yourself, especially limewater from a reservoir. If you aren’t handy, then by all means buy a complete system.

Medium Tanks

These are the tanks that most beginners have, containing 30-90 gallons. The logical choices to pick from are more numerous than for the systems described above, and will come down to a series of different factors.

Does the tank have a sump where you can dispense additives with less concern about nearby corals? If so, that’s a plus for limewater, Biocalcium, and the high pH two-part additives (including DIY and Balling).

Do you have a good skimmer or other source of aeration? If so, that is a plus for using limewater (that needs to suck CO2 from the air) or a calcium carbonate/carbon dioxide reactor (that needs to blow off excess CO2). If not, using Biocalcium, All in One, or a two-part additive that has a small pH effect (like the bicarbonate B-ionic or a DIY two/three part using sodium bicarbonate) may be better choices.

Is your home very tightly sealed, with possibly high indoor CO2? That is a plus for using limewater or a high pH two-part additive as they will counteract the tendency toward low pH.

Do you have space under or behind the tank for equipment? If so, that is a plus for limewater or CaCO3/CO2 reactors that need space and are typically unattractive. If not, that is a big plus for the simpler additives (two-part systems, All in One, Biocalcium).

Are you handy with complex systems? That is a plus for CaCO3/CO2 reactor systems and complicated auto top-off systems using limewater. If not, that is a plus for the simpler systems.

Are you very concerned about copper or phosphate in your tank from your additives? If so, perhaps pick limewater.

Are you going to leave the tank unattended for more than a few days? Pick a system with automatic delivery (many can be automated with the right selection of appropriate equipment, except Biocalcium).

Will the tank have a very high demand for calcium and alkalinity? That is, will it have lots of fast growing corals? If so, a CaCO3/CO2 reactor could be the best choice.

How much is cost an issue? For lowest cost, a cheap limewater drip will probably be best, followed by a DIY two-part.

Of course there are many other issues to consider, and most of these were described in the body of the article. If you are just setting up a tank for the first time, I’d advise looking at existing tanks, and deciding what you want in it first. Then look to see what kinds of supplementation schemes these tanks use, and ask the owner how it is working out, and actually see for yourself what it looks like and what is involved. Then you’ll be in a good position to make an informed choice.

Dosing Instructions

My generalized dosing instructions are basically the same for any balanced method, but how you adjust the dose is obviously very different.

To initiate dosing, first adjust calcium and alkalinity to roughly their correct ranges. This may require a dose of calcium chloride if calcium is low (e.g., below 400 ppm). Low alkalinity is not generally an issue because it can be made up with the balanced method (low calcium cannot).

I would suggest targeting calcium between 380 and 450 ppm, and alkalinity between 7 and 11 dKH (2.5 -4 meq/L ; 125-200 ppm calcium carbonate equivalents) unless it is an Ultra Low Nutrient System (ULNS), where alkalinity in the 7-8 dKH range may be more appropriate..

Then, once things seem roughly correct, select a starting dose for routine dosing. After a few days of dosing, note whether alkalinity is low, high or on target. You only need to test alkalinity, not calcium, during this period, because it is much more sensitive than calcium to over- or under-dosing of any balanced method. Adjust the dose up or down as necessary to increase or decrease the alkalinity.

Once you have determined the proper dose to maintain alkalinity where you want it, continue it until there is a substantial reason to adjust it (such as falling alkalinity as the corals increase in size, or a water changes messes with your levels).

Resist the temptation to keep jiggering calcium and alkalinity independently. They will need occasional corrections, but that should not be the normal course of dosing unless there are substantial outside influences, such as water changes with a salt mix that does not match the tank’s parameters or an error in making the products.

Check alkalinity fairly frequently to make sure the dosing continues at a suitable rate. Check it maybe once a week to once a month (or less as you get more experienced with the system and the tank). Check calcium once a month to once every few months to make sure it continues on track. It should follow along just fine using any of these balanced methods (assuming you are not otherwise adding calcium or alkalinity somehow).

Good Luck and Happy Reefing!

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