Calcium and alkalinity are supplied to reef aquaria in order to balance the losses caused by the formation of calcium carbonate. This formation takes place in hard corals to form their skeletons, and in other internal structures such as spicules in certain soft corals. It also takes place in a wide range of other organisms, ranging from coralline algae to snails to clams. Deposition of calcium carbonate also takes place outside of biological systems, such as on heaters and pump impellers, where the increased temperature results in decreased solubility of calcium carbonate, and hence a greater likelihood of precipitation.

In each of these cases, what is being deposited is largely calcium carbonate. Since calcium and carbonate are present in pure calcium carbonate at exactly equal concentrations (one ion of calcium to one ion of carbonate), the removal rate of calcium and carbonate by all of the mechanisms described above should be the same. To a great extent, aquarists use alkalinity as a surrogate measure of carbonate (and bicarbonate). The exact balance between calcium and carbonate demand in a reef aquarium is therefore equally well described as a balance between calcium and alkalinity demand.

Reef aquarists take great advantage of the 1:1 matching of calcium and alkalinity demand in reef aquaria by using additives that supply calcium and alkalinity in this same ratio. In this way, over- or under-dosing of such balanced calcium and alkalinity additives should not result in skewing the aquarium water's chemistry toward too much calcium and too little alkalinity, or too much alkalinity and too little calcium. On the other hand, independent additions of calcium and alkalinity, even with careful and frequent measurement, often lead to such imbalances.

There exist a variety of such balanced additives, and many have been described and compared in detail in previous articles. They include calcium carbonate/carbon dioxide (CaCO3/CO2) reactors, limewater (kalkwasser), the two part additive systems, and some one-part systems. As a class, I strongly recommend them over any other unbalanced additive method for most reef aquarists.

There are, however, several reasons that calcium and alkalinity balance is not always perfect. In many reef aquaria using only balanced additive systems, the levels will slowly drift away from perfect balance, and will require occasional correction. Whether this correction is needed monthly or yearly, and in what direction, will depend on the system's details. Before discussing these real calcium and alkalinity demand imbalance issues, I will also describe one "mechanism" that confounds many aquarists by appearing to represent a drift in the balance, but that really does not. The "mechanism" arises in the simple fact that alkalinity rises and falls much faster than does calcium because seawater has a much bigger reservoir of calcium than it does alkalinity.

This article will describe the various mechanisms that cause such drift, and will quantify the magnitude of each effect. These mechanisms include:

1. Incorporation of magnesium and strontium in place of calcium in deposited calcium carbonate.

2. Reduction in alkalinity through partial completion of the nitrogen cycle.

3. Changes in the calcium and alkalinity balance through water changes.

4. Additions of calcium or alkalinity via top-off water.

Understanding these various factors will take some of the mystery out of reef aquarium chemistry, and will allow aquarists to become masters of their aquarium chemistry, rather than slaves to "unpredictable" changes.

Calcium and Alkalinity Demand: Calcium Carbonate Mathematics

Calcium carbonate formation consumes its two components in an exact 1:1 ratio. In the units used by aquarists, this ratio corresponds to one meq/L (2.8 dKH; 50 ppm CaCO3 equivalents) for every 20 ppm of calcium. Not surprisingly, this is also the ratio of alkalinity to calcium that is supplied when calcium carbonate is dissolved, as in a CaCO3/CO2 reactor. Fortuitously for the aquarist, this is also the ratio supplied when calcium hydroxide is dissolved, as with the use of limewater (kalkwasser).

Apparent Excess Demand for Alkalinity

One of the most common complaints of new aquarists is that their aquaria seem to need more alkalinity than their balanced additive system, such as limewater, is supplying. While there are reasons this may actually be the case over the long term (these will be detailed later in this article), frequently these aquarists are seeing a "chemical mirage" rather than a real excess demand for alkalinity.

One of the interesting features of seawater is that it contains a lot more calcium than alkalinity. By this I mean that if all of the calcium in seawater (420 ppm; 10.5 meq/L) were to be precipitated as calcium carbonate, it would consume 21 meq/L of alkalinity (nearly 10 times as much as is present in natural seawater). In a less drastic scenario, let's say that calcium carbonate is formed from aquarium water starting with an alkalinity of 3 meq/L that it is allowed to drop to 2 meq/L (a 33% drop). How much has the calcium declined? It is a surprise to many people to learn that the calcium would drop by only 20 ppm (5%). Consequently, many aquarists observe that their calcium levels are relatively stable (within their ability to reproducibly test it), but alkalinity can vary up and down substantially. This is exactly what would be expected, given that the aquarium already has such a large reservoir of calcium.

So the first "deviation" from the rule of calcium and alkalinity balance really isn't a deviation at all. If an aquarist is supplying a balanced additive to his aquarium, and calcium seems stable but alkalinity is declining, it may very well be that what is needed is more of the balanced additive, not just alkalinity. This scenario should be assumed as the most likely explanation for most aquarists who should look for more esoteric explanations for alkalinity decline only if calcium RISES substantially while alkalinity falls. Likewise, if alkalinity is rising and calcium seems stable when using a balanced calcium and alkalinity additive system, the most likely explanation is that too much of the additive system is being used.

The real imbalance effects described later in this article take effect slowly, and are manifested over weeks, months and years. This short term "chemical mirage" caused simply by the mathematics of calcium and alkalinity additions can be seen in a single addition. Any effect that develops rapidly over the course of a few days is almost certainly not a true imbalance.

The following scenarios show what can happen to a reef aquarium whose dosage with a balanced additive system does not match its demand. Table 1 shows what can happen when the dosing is inadequate. Alkalinity drops fairly rapidly. After two days, many aquarists might conclude that they need additional alkalinity, when in reality, they need more of both calcium and alkalinity to stabilize the system.

Table 2 shows what happens when too much of a balanced additive is added. After a few days, many aquarists would conclude that alkalinity is rising too much, but that calcium is fairly stable. Again, what is needed is less of the balanced additive, not just less alkalinity.

Real Excess Demand for Alkalinity: Magnesium and Strontium

Many sharp aquarists will correctly dispute the notion I professed in the introduction, that calcium and alkalinity are exactly balanced, because coral skeletons are not pure calcium carbonate. In fact, they contain significant amounts of magnesium and strontium. Abiotically precipitated calcium carbonate also contains such ions. In short, magnesium and strontium enter the calcium carbonate structure in place of calcium, reducing the amount of calcium required for a given amount of carbonate. Consequently, the aquarium is skewed toward less calcium demand and more towards alkalinity demand for this reason.

How big is this effect? In terms of magnesium, it is hard to say exactly how big the effect will be because the amount of magnesium deposited depends on the species involved, and ranges from less than 1% magnesium by weight in the skeleton, to more than 4%. Consequently, the magnesium demand in one aquarium may be very different from the magnesium demand in a second aquarium whose calcium demand is exactly the same.

Figure 3. This photograph of the underside of coralline algae was taken by Bob Bottini (aquababy) owner of Tanks alot!

Nevertheless, we can roughly calculate the magnitude of the effect. Depositing pure calcium carbonate requires 20 ppm of calcium for every 1 meq/L of alkalinity. Substituting magnesium to the extent of 1% by weight in the skeleton decreases the calcium content by 4.1%. So the demand is then 19.2 ppm calcium for every 1 meq/L of alkalinity. Substituting magnesium to the extent of 4% by weight in the skeleton decreases the calcium content by 16.5%. So the demand is then 16.7 ppm calcium for every 1 meq/L of alkalinity. The change in the balance of the demand caused by magnesium incorporation into corals will depend on the exact species driving the demand, but can be larger than the other causes described in this article.

Strontium has a rather smaller effect. Corals, coralline algae, and abiotically precipitated calcium carbonate in natural seawater typically have roughly one strontium ion for every 100 calcium ions (whether these are dispersed within calcium carbonate, or as a separate strontium carbonate phase). In a reef aquarium, where the strontium level can be twice the natural level, this strontium incorporation can be higher, on the order of one strontium ion for every 50 calcium ions. The replacement of calcium by strontium in the carbonate crystals has the effect of reducing the calcium demand from 20 ppm per one meq/L of alkalinity to 19.8 meq/l for natural levels of strontium, and to 19.6 ppm at double the natural level. This strontium effect is smaller than the magnesium effect, but can be comparable to the other effects described in this article. In addition, the amount of substitution by ions other than calcium in forming carbonates may depend on other factors, including temperature and (as with the strontium example above) the relative concentrations of the ions present.

Alkalinity Decline in the Nitrogen Cycle

One of the best known chemical cycles in aquaria is the nitrogen cycle. In it, ammonia excreted by fish and other organisms is converted into nitrate. This conversion produces acid, H⁺ (or uses alkalinity depending on how one chooses to look at it), as shown in equation 1:

(1)  NH₃   + 2O₂  à  NO₃^-  +  H⁺  + H₂O

For each ammonia molecule converted into nitrate, one hydrogen ion (H⁺) is produced. If nitrate is allowed to accumulate to 50 ppm, the addition of this acid will deplete 0.8 meq/L (2.3 dKH) of alkalinity.

However, the news is not all bad. When this nitrate proceeds further along the nitrogen cycle, depleted alkalinity is returned in exactly the amount lost. For example, if the nitrate is allowed to be converted into N2 in a sand bed, one of the products is bicarbonate, as shown in equation 2 (below) for the breakdown of glucose and nitrate under typical anoxic conditions as might happen in a deep sand bed:

(2)  4NO₃^-  +  5/6 C₆H₁₂O₆  (glucose)  +   4H₂O   à  2 N₂  + 7H₂O + 4HCO₃^- +  CO₂

In equation 2 we see that exactly one bicarbonate ion is produced for each nitrate ion consumed. Consequently, the alkalinity gain is 0.8 meq/L (2.3 dKH) for every 50 ppm of nitrate consumed.

Likewise, equation 3 (below) shows the uptake of nitrate and CO2 into macroalgae to form typical organic molecules:

(3)  122 CO₂ + 122 H₂O + 16 NO₃^-    à  C₁₀₆H₂₆₀O₁₀₆N₁₆  + 138 O₂  +  16 HCO₃^-

Again, one bicarbonate ion is produced for each nitrate ion consumed.

It turns out that as long as the nitrate concentration is stable, regardless of its actual value, there is no ongoing net depletion of alkalinity. Of course, alkalinity was depleted to reach that value, but once it stabilizes, there is no continuing alkalinity depletion because the export processes described above are exactly balancing the depletion from nitrification (the conversion of ammonia to nitrate).

There are, however, circumstances where the alkalinity is lost in the conversion of ammonia to nitrate, and is never returned. The most likely scenario to be important in reef aquaria is when nitrate is removed through water changes. In that case, each water change takes out some nitrate, and if the system produces nitrate to get back to some stable level, the alkalinity again becomes depleted.

Figure 4. Porites species vary with respect to the amount of magnesium incorporated, from less than 0.1% to over 1% magnesium in the skeleton. Photo courtesy of Skip Attix.

If, for example, nitrate averages 50 ppm at each water change, then over the course of a year with 10 water changes of 20% each, the alkalinity will be depleted by 1.6 meq/L (4.5 dKH) over the course of that entire time period. This process is one of the primary reasons that fish-only aquaria that often export nitrate in water changes need occasional buffer additions to replace that depleted alkalinity.

While the magnitude of the depletion described in the paragraph above is fairly easy to understand, it also can be converted into units that clarify the imbalance. The impact of alkalinity depletion on the calcium and alkalinity demand balance depends, of course, on the amount of calcium and alkalinity added (and consumed) over the course of that same year.

For a typical reef aquarium (assuming a daily addition of saturated limewater equal to 2% of the tank's volume), the amount of alkalinity added during the course of a year is 297.8 meq/L. Likewise, the amount of calcium added is 5,957 ppm Ca⁺⁺, given the ratio of 1 meq/L of alkalinity for every 20 ppm of calcium that has been discussed above. If that 1.6 meq/L of alkalinity is added to create a larger demand of 299.4 meq/L over the course of a year, the new ratio for the total demand becomes 19.90 ppm Ca⁺⁺ per 1 meq/L of alkalinity. Consequently, while this effect of nitrate production on alkalinity is enough to be noticed over the course of a year, it is substantially smaller than the other effects discussed in this article, and is unimportant for aquaria that maintain low nitrate levels.

Effects Due to Water Changes

Another reason that calcium and alkalinity demand is not exactly balanced in many aquaria has to do with water changes. Many aquarists (including myself) do not attempt to match the calcium and alkalinity levels in water change water to the aquarium water. Consequently, each water change will alter these levels in the aquarium, and will alter the observed balance between calcium and alkalinity demand. What direction the change takes, however, will depend on the salt mix chosen and the aquarium water parameters. Commercial salt mixes vary from high calcium and normal alkalinity to high alkalinity and low calcium.

For example, if the aquarium is maintained at 420 ppm calcium and 4 meq/L of alkalinity, and the water change has 500 ppm calcium and 2.5 meq/L of alkalinity, each 20% water change will increase calcium by 16 ppm, and will drop alkalinity by 0.3 meq/L. Using the same water change scenario used in the nitrate calculations above (10 changes of 20% each over the course of a year), these water changes will increase calcium by 160 ppm and drop alkalinity by 3 meq/L.

Figure 5. While many soft corals do use calcium and alkalinity to form internal structures made from calcium carbonate, Xenia seems to have few if any such structures. Consequently, it does not significantly impact the demand for calcium or alkalinity in reef aquaria. Photo courtesy of Gregory (www.ximinasphotography.com).

For a typical aquarium (assuming a daily addition of saturated limewater equal to 2% of the tank's volume), the amount of alkalinity added during the course of a year is 297.8 meq/L. Likewise, the amount of calcium added is 5,957 ppm Ca⁺⁺, given the ratio of 1 meq/L of alkalinity for every 20 ppm of calcium that has been discussed above. If that amount of alkalinity demand is increased by 3 meq/L to 300.8 meq/L over the course of a year, and the calcium demand is decreased by 160 ppm to 5797 ppm, the new ratio for the total demand becomes 19.30 ppm Ca⁺⁺ per 1 meq/L of alkalinity. Consequently, the effect of water changes can be significant, but will depend entirely on how much the aquarium water deviates from the water change water, and on the amount of water changed.

The Effect of Top-Off Water

A final factor that can impact the apparent calcium and alkalinity demand is the possibility of delivering calcium or alkalinity or both in top-off water. Water that is purified by reverse osmosis (RO) followed by deionization (DI), and water that is purified by distillation will not deliver any significant amount of calcium or alkalinity (regardless of the apparent pH when such a measurement is taken). The same is true for water purified by DI only. Water purified by only RO may have a small amount of calcium or alkalinity in it, depending on the nature of the source water.

The greatest chance for effects to tankwater calcium and alkalinity levels comes from the use of tap water or spring water (neither of which I recommend for reef aquaria). In a recent article describing concerns with the use of tap water in reef aquaria, I showed that water from municipal water supplies can range from 0 to 93 ppm calcium and 0 to 5.5 meq/L alkalinity. Obviously, tap water with close to zero calcium and alkalinity will not appreciably impact the calcium and alkalinity balance. At the extremes, however, these values can have a large impact.

If we assume that an aquarium receives 2% of its tank volume daily to replace evaporated water, then one extreme is a case where over the course of a year, 679 ppm of calcium is added, and no alkalinity. At the other extreme, 40 meq/l of alkalinity is added, and no calcium.

For a typical aquarium (using 2% of the tank volume daily in saturated limewater), the amount of alkalinity added during the course of a year is 297.8 meq/L. Likewise, the amount of calcium added is 5,957 ppm Ca⁺⁺, given the demand ratio of 1 meq/L of alkalinity for every 20 ppm of calcium discussed above. If that amount of alkalinity demand is decreased by 40 meq/L, due to alkalinity in tap water, to 257.4 meq/L over the course of a year, and the calcium demand is unchanged, the new ratio for the total apparent demand becomes 23.1 ppm Ca⁺⁺ per 1 meq/L of alkalinity. Likewise, if the calcium demand is decreased by 679 ppm of calcium, to 5278 ppm, the new apparent demand ratio becomes 17.7 ppm Ca⁺⁺ per 1 meq/L of alkalinity. These extreme cases may not actually happen anywhere, since the extreme case for calcium and the extreme case for alkalinity occur in the same city (Kansas City in 2003), so they partially offset each other. Nevertheless, the effect easily could be half as large in many cities, and it is apparent that this effect of tap water can be significant, and may even dominate the other effects.

Other Effects

Other effects may also skew the demand for calcium and alkalinity in aquaria. These include foods that contain calcium or, rarely, alkalinity, and various additives that aquarists use. Most additives do not contain alkalinity (except, of course, buffers and anything claiming to control pH or supply alkalinity), although sodium silicate and borax (borate) do provide alkalinity. Since many additives do not even say what they contain, it is hard to say what effect they might have, but I'd expect most of them to be inconsequential in this respect.

How Are Additive Systems Really Balanced?

Since, for the reasons described above, the demand for calcium and alkalinity may not be precisely balanced at 20 ppm calcium per 1 meq/l of alkalinity (matching pure calcium carbonate formation), the question arises, what ratio is used in balanced additive systems?

According to the ESV web site, the two part system B-ionic has a balance of 19.3 ppm calcium per 1 meq/L of alkalinity. That value is probably a fine balance for the calcium and alkalinity ions given the effects of magnesium and strontium incorporation. Other brands that aquarists frequently use do not give adequately detailed information about their products to show what the exact ratio might be.

Clear, settled limewater has a ratio of approximately 20.0 ppm Ca⁺⁺ to 1 meq/L of alkalinity. It has no significant magnesium in it, and its strontium level is very low. For those dosing cloudy limewater, lime solids that I have measured contain enough magnesium to drop the ratio to about 19.9 ppm calcium per 1 meq/L of alkalinity.

Koralith CaCO3/CO2 reactor media has slightly less magnesium and strontium than does the lime that I tested, and would have a ratio of 19.9 ppm calcium per 1 meq/L of alkalinity. A different brand of media, Super Calc Gold, has more magnesium, with a resulting ratio of about 19.8 ppm calcium per 1 meq/L of alkalinity. A third brand, Nature's Ocean crushed coral, has a similar level of magnesium, resulting in a ratio of 19.8 ppm calcium per 1 meq/L of alkalinity. All of these brands may fall short of the rate of incorporation of magnesium in reef aquaria, as has been discussed in previous articles. Some aquarists have taken to adding a small amount of dolomite (a material containing both calcium and magnesium carbonates) to their CaCO3/CO2 reactors to add an appropriate amount of magnesium.

Which Mechanisms Predominate in Reef Aquaria?

Mechanisms resulting in a deviation from an exact balance between calcium and alkalinity will obviously vary between aquaria with different calcifying species and with different husbandry practices. Some of the mechanisms may have opposite effects on the balance, partially canceling each other out in some aquaria (e.g., water changes with a high alkalinity/low calcium salt mix vs. magnesium and strontium incorporation). Consequently, it isn't possible to say which effect will dominate reef aquaria in general.

Figure 6. Tridacna species of clams deposit calcium carbonate in their shells, and can be a significant source of calcium and alkalinity demand in reef aquaria with many clams. Photo courtesy of Gregory (www.ximinasphotography.com).

In my aquarium, using limewater, I do enough water changes that over time my aquarium keeps a balance that is similar to that found in the Instant Ocean salt mix that I use for water changes. After running this aquarium for about 10 months after I took the last calcium and alkalinity measurement, the levels were about 3.6 meq/L for alkalinity and 360 ppm for calcium. At that point, I raised the calcium to about 420 ppm with calcium chloride. As I have stated in previous articles regarding magnesium in my aquarium, I am not certain why the demand for magnesium (and its effect on the calcium and alkalinity demand) is not greater.

Conclusion

A variety of reasons prevent reef aquarists from experiencing exactly balanced demands for calcium and alkalinity. These include the effects of incorporation of magnesium and strontium into coral skeletons, the effects of water changes with new water that does not match the aquarium water in terms of chemical ions present, and top off water that contains calcium or alkalinity. Aquarists also may be sometimes fooled into thinking they are seeing imbalanced demand when in reality they are simply observing the fact that on a percentage basis, alkalinity goes up and down much faster than calcium. Understanding how and when these differences arise will allow reef aquarists to better deal with them, and not take inappropriate actions to "correct" them.

Happy Reefing!