Let me begin by saying that I am not a chemist. Indeed, (and this may become obvious), I have not even attended a chemistry class of any kind. This is a distinct weakness in that my understanding of the complex chemical interactions involved in water parameters for the reef aquarium has been slow to advance, and is still far from complete. However, it may be a strength in that I may be able to explain, in layman's terms, some important concepts involved in the use of calcium in the reef tank. To those of you who are more chemically knowledgeable, I apologize for my overly simplistic terms and descriptions. To those of you just learning these concepts, I hope I can be of some help. It is my intent to use plain language, as much as is possible, so that even the newest of hobbyist can use this information in deciding how to add calcium to their own system.

Calcium on the Reef
Calcium is a crucial element on the natural reef, and in fact is a building block of the reef itself. Many corals, as well as invertebrates, clams, desirable algaes, and other creatures on the reef depend on adequate calcium being available to them in the water. This is provided by an elaborate dance in the ecosystem whereby coral skeletons, and other calcium rich materials, are broken down into bio-available calcium in the water. This system is highly complex and multifaceted, so mimicking this in our reef aquariums is difficult. However, this does not mean that we are unable to provide an adequate calcium environment in our reef aquariums.

Calcium in the Reef Aquarium
In trying to determine what is considered a "good" calcium level for my tanks, I found a number of authors who mention various levels of calcium that are found on the reef. I also found a variety of suggested levels of calcium for reef aquariums. It seems to be generally accepted that the calcium level in the reef aquarium should be somewhere between 350 ppm (parts per million) and 450 ppm.

Calcium levels that exceed approximately 550 ppm in the typical reef setup will "precipitate" out of solution. This means that given the water conditions at the time (including elements such as pH, temperature, and alkalinity) the calcium is saturated in the water and no more can be held. At this saturation point, the calcium becomes solid and forms calcium carbonate. When this happens, the tank can appear as if it is "snowing", or may become quite cloudy. It would be logical to think that this calcium carbonate is indeed "calcium", so what's wrong with this? The problem is that this particular form of calcium is not available in a biologically acceptable form to the tank inhabitants.

If calcium is not added to the aquarium in some form, calcium levels will fall naturally as the calcium is used by the tank inhabitants. When calcium levels fall below 350 ppm, there is no calamity. Indeed, reef aquariums that have calcium levels below 250 ppm are not uncommon, and the vast majority of inhabitants are not generally in imminent danger at these low levels. However, specimens may not thrive and desirable algaes may not grow well. It is therefore, desirable to try and maintain adequate calcium levels in the aquarium to promote properly coral health and colorful coralline algaes.

Calcium and Alkalinity, pH, Salinity and Magnesium
To the new hobbyist, these terms can be rather intimidating. And when you are trying to consider the relative effects of each on the other, it can become quite confounding. I will try and cover some of the basic elements of the interaction of these elements as it relates to calcium.

PH is simply a measure of the relative balance of hydroxide and hydrogen ions in water. In layman's terms, this means that there are hydroxide elements and hydrogen elements in the water. In a particular solution, whatever the balance is in these two elements determines the "pH". PH is measured on a scale from 0 to 14, with the midpoint, 7 being described as "neutral". Solutions with a pH measurement above 7 are considered "base", while those below 7 are considered "acidic". Seawater pH is typically 8.0 - 8.5, and is therefore somewhat "base". Another thing to note about pH, is that the scale is logarithmic. This means is that each gradation represents a factor of ten. So a pH of 8 is 10 times more base than 7, and 9 is 100 times more base than 7, and so on.

What is important about all this is that small changes in pH are very significant to tank inhabitants and to calcium levels. With all things being equal, the higher the pH of the water, the less calcium the water will hold "in solution". This is not generally a problem as long as the hobbyist keeps the tank pH within the prescribed range, 8.0 - 8.5. However, some methods of adding calcium will affect aquarium pH, which is why it is important to understand the relationship of calcium and pH.

Alkalinity, in short, is the water's ability to withstand additions of acids, without changing the pH. Other terms that are commonly used to describe alkalinity include buffer or buffering, DKH, meq./ltr., and carbonate hardness. For all practical purposes, they are all the same thing (there are some slight differences in some, but not enough for our purposes here). The elements that provide this effect are carbonates, mostly sodium carbonate and sodium bicarbonate. Alkalinity is necessary in the aquarium to help guard against large swings in pH, which will stress the tank inhabitants. Recommended levels range from 7 dkh to 12 dkh. The other commonly used scale to measure alkalinity is meq./ltr (read as megaequivalents per litre). To convert dkh to meq./ltr you simply divide by 2.8. Relative to calcium, the higher the dkh, the less calcium that will stay in solution. So long as alkalinity is kept in the acceptable range, this should not hinder achieving proper levels of calcium. But, like pH, some calcium additive methods affect alkalinity, thus this basic understanding.

I would also like to refer those who are curious about the calcium/alkalinity dance to an article written by Craig Bingman (Aquarium Frontiers on-line, July 1998). This is an excellent discussion of the relationship between alkalinity and calcium, and recommendations regarding simulating natural seawater levels of both components.

Salinity is another factor in calcium concentrations. The above calcium recommendations are at full strength seawater with a specific gravity of 1.025. If the aquarium is kept at a lower specific gravity, then a given concentration of calcium measured in the water will really be at a higher relative concentration to the other water elements. Put simply, if your tank is run at lower than full strength seawater levels, then the required concentration of calcium will also be proportionally lower.

Magnesium is another player in the dance with calcium. Magnesium levels are typically pretty stable in the aquarium, and are easily maintained with routine water changes. However, if calcium precipitation occurs, or the hobbyist does not do regular water changes, magnesium levels can decline. The recommended level of magnesium in the aquarium is 1300 ppm to 1500 ppm. If magnesium levels do fall, calcium has been found to be much more difficult to get to the proper levels of concentration. However, I want to emphasize that when the hobbyist encounters difficulty maintaining calcium levels, it is best to rule out all other causes before considering magnesium deficiency. This is simply because it is far more often the case that other factors are at play in the problem. If it is suspected that magnesium deficiencies are a problem, it is recommended that a magnesium test kit be purchased and used to verify the problem. Magnesium supplements are readily available, and replenishment can also be accomplished through the use of Epsom Salts.

Methods of Calcium Additions
This list of methods of adding calcium is not meant to be comprehensive or complete. It is, to my knowledge, the most common methods employed by average hobbyist. I've attempted to include a description of the method, it's benefits and it's risks, so that an informed decision can be made about methods to employ. I want to stress that there is no "correct" method. All have there up sides, and down sides. The evaluation of these is from my personal perspective, and is not meant as an endorsement or slam at manufacturers, products or methods. I strongly encourage anyone considering such a decision to do lots of investigation about the methodology that is most likely to be of benefit to them, and their aquarium inhabitants. There are a number of high quality books on reef keeping that contain more comprehensive information that I am providing here, and it is encouraged that these be consulted as well. As a final disclaimer, it is very important that the consumer carefully read the package/label instructions of a particular product prior to its use. The labels and "fine" print contain much useful information that can help the aquarist avoid costly mistakes.

When anyone embarks upon the attempt to increase and maintain calcium levels in the reef aquarium, it is important to note that all methods are best used slowly. That is, increasing calcium levels in the aquarium is best done gradually and over time, no matter which method you choose. This not only helps the tank inhabitants to adjust to changes in water parameters, but also greatly helps avoids problems that typically occur with each particular method.

Water Changes:
Probably the original and simplest method of calcium replenishment is with regular water changes. Most commercial salt mixes contain calcium levels that exceed 350 ppm. With low aquarium calcium usage, and frequent large water changes, calcium can be successfully maintained by simply removing aquarium water, and replacing it with fresh mixed salt water.

The primary advantage of this method is that it is easy to do, and doesn't require much in the way of water chemistry knowledge in order to accomplish it. The primary disadvantages are that it if calcium usage goes up, salt mixes can become expensive and water changes can become more time consuming. It must also be noted that it is not unusual on tanks with high levels of calcium, that when new salt water is introduced into the aquarium, calcium may precipitate out of solution due to a dramatic increase in calcium. This is often seen as "cloudiness" after a water change. It also appears that there is some variation in calcium concentrations between various commercial salt mixes. There is even variation within a variety of mixes from a particular manufacturer. It is therefore encouraged that new salt mixes be tested for calcium concentration, particularly if this method is to be relied upon for calcium supplementation.

Kalkwasser:
Kalkwasser, or "limewater" is actually calcium hydroxide, and derives its name from its German roots. It is a very fine powder, much like talcum powder, that is slightly caustic and should be handled with care. It is most often introduced into the tank via makeup water, that is, the water added to the tank to make up for any water that has evaporated from the aquarium. There are two basic methods of adding the kalkwasser, through slow dosing/dripping into the water stream, or through the use of a kalkreactor.

When kalkwasser is dosed or dripped into the tank, the following procedure is more or less used. One to three teaspoons of kalkwasser are added to a gallon of water that is to be used for makeup water. The kalkwasser is then gently mixed to saturate the water with the kalkwasser. I say gently, because an important consideration in using kalkwasser is to minimize the amount of air that enters the water during mixing. When kalkwasser is combined with highly aerated water, it forms calcium carbonate, which is not useful in the aquarium. After the gentle mixing, the kalkwasser mix should then sit for a period of a few hours to allow settling of any calcium carbonate that has formed. This, along with some unmixed kalkwasser, will form sediment on the bottom of the container. After settling, the saturated kalkwasser (the liquid above the sediment) is siphoned off to be used for dosing. (More on dosing in a minute.) When using this method of kalkwasser addition, it is important to use the mixed kalkwasser within a few days. Old kalkwasser mix will lose its concentration of calcium in solution, which results in little calcium addition to the aquarium. And it is important to not dose the sediment, which is a mixture of calcium carbonate and calcium hydroxide. Dosing this can result in calcium precipitation, excessive elevation of pH, and decline of alkalinity.

The other method, and generally more preferable method of mixing and dosing kalkwasser, is through the use of a kalkreactor. These devices may go under the name of a calcium reactor, kalkreactor, and Nilsen reactor, to name a few. In fact, there is another type of reactor (to be discussed later) that is also called a calcium reactor that is quite a different device. For the sake of this discussion, I will use the term "kalkreactor". A diagram of this device may be found in Sprung and Delbeek's "The Reef Aquarium, Vol. I", pg. 245. There are also a number of do-it-yourself (DIY) projects on the Internet for building one of these. Essentially, a supply of kalkwasser is inserted into a sealed reactor chamber. Within the chamber is a magnetic stirrer/spinner. Using a dosing pump, water is drawn from a fresh water reservoir and pumped into the reactor chamber, where the magnetic stirrer periodically mixes the water and kalkwasser. The pressure build-up from the dosing pump within the reactor forces the saturated and milky kalkwasser then to be dosed into the aquarium. The advantage of this method is that the calcium concentration of the kalkwasser remains higher, and the mixing procedure requires far less attention.

The procedure for dosing kalkwasser is important. A saturated kalkwasser mix will have a pH that can exceed 12. If added too quickly, the aquarium pH can rise significantly, which stresses the tank inhabitants. Beyond this, a significant pH spike (usually above 8.5 - 8.7) can cause a calcium/carbonate/magnesium precipitation. This leaves the tank with stressed livestock and often less calcium than before dosing. Ideally, a pH monitor and/or controller should be used when dosing kalkwasser to insure that tank pH remains below 8.5, however this is not a requirement. If dosing kalkwasser without a pH monitor, it is important to drip it slowly (around 1 drop/second maximum) into a water stream in a sump or in the tank itself.

The disadvantages of kalkwasser may be fairly apparent. If mixing it for makeup water, it can take time and diligence to mix it properly, and dose it properly. A kalkreactor greatly reduces the inconvenience, and can provide a highly saturated kalkwasser mix. Another disadvantage is that kalkwasser additions are limited to the amount of water that is used to replenish from evaporation. Many complain that they just can't get the calcium levels up using kalkwasser alone. Often it is cited that kalkwasser is a highly effective method for maintaining calcium levels that have been obtained by the use of other methods, particularly by those who mix their own kalkwasser. Those utilizing kalkreactors report that they are able to build calcium levels, as well as maintain them in a relatively trouble-free way.

The advantages of using kalkwasser are many. First, kalkwasser, when mixed properly, is highly saturated in calcium, and leaves no other residues in the aquarium. This accomplishes the primary goal of kalkwasser use, namely increasing calcium availability to aquarium inhabitants. Kalkwasser helps to maintain alkalinity in the aquarium. On this point, I want to highlight that kalkwasser does not increase alkalinity, but rather helps maintain alkalinity by neutralizing acids that would usually be neutralized by resident buffering agents. The result is that carbonates in the aquarium, and thus alkalinity, is preserved. Kalkwasser helps maintain pH. This occurs not only due to the preservation of alkalinity, but also because of the high pH of kalkwasser. When properly dosed, kalkwasser can be used as a tool to gently increase pH to more desirable levels in the aquarium. This is useful in combating the natural trend in closed systems of pH decline. Finally, kalkwasser is known to precipitate phosphates out of solution. What this means is that when calcium hydroxide enters the water, it causes phosphates to fall out of solution. This is helpful in preventing/removing problem causing algae and cynobacteria blooms.

Calcium Chloride:
Calcium chloride is sold by a number of manufacturers as a method of increasing calcium in reef aquariums. It is usually sold in liquid form, but can also be found in powder form. The most prominent of these products is Kent's "Liquid Calcium" and Kent's "Turbo Calcium". However, there are a number of manufacturers who utilize this formulation for calcium supplementation. Typically, the product is mixed with a quart or so of aquarium water, then slowly poured into the aquarium. After a few hours, a reading may be taken, then additional calcium dosed. The obvious advantage of calcium chloride is its ease of use. It dissolves readily in water, and can be poured into the aquarium without affecting pH. It is also highly concentrated, so when added properly it can raise calcium levels relatively rapidly.

The disadvantages include that adding it too quickly can cause microenvironments where calcium saturation occurs, causing calcium and carbonates to fall out of solution. So when added, it must be dosed according to label instructions and poured slowly.

Another disadvantage is that the use of calcium chloride will cause a loss of alkalinity. Therefore, when dosing calcium chloride, it is important to routinely measure the alkalinity of the aquarium, and add buffer to maintain alkalinity levels. If either calcium chloride, or buffer, is adding too quickly or in too large of a concentration at once, precipitation can occur which results in lowered alkalinity AND lower calcium levels. This often results in a seesaw experience of dosing one, then the other, to help maintain proper tank parameters. This experience can lead to further frustration and impatience unless the mechanisms at work are understood.

A final disadvantage of calcium chloride use is the buildup of chlorine in the aquarium. If regular water changes are not done, and calcium chloride is used, chlorine ions can accumulate in the system, causing harm to the livestock. Admittedly, this is a rare occurrence as most hobbyist do routine water changes.

Balanced Calcium Additives:
A number of manufacturer's, primarily in response to the frustration encountered by the use of calcium chloride, developed calcium additives that are "balanced". In this case, balance refers to the product's ability to raise/maintain calcium levels while also maintaining alkalinity levels. Several manufacturer's, including Kent and ESV sell these products. They are typically in liquid form, and in a two-part solution (two separate bottles). The "part A" liquid is dosed first, then an equal amount of a "part B" is dosed. The dosing amount can be adjusted based on the aquarium's calcium usage, with daily dosing limits posted on the label.

The advantage of these products is the ease of use. You simply pour a measured amount of each part into the aquarium each day.

One of the disadvantages of this method is that the amount that may be dosed at one time is somewhat limited by the fact that the calcium portion of the solution tends to increase pH. If aquarium pH exceeds approximately 8.5, then calcium may precipitate out of solution, causing little if any calcium increase. So while it can be poured into the aquarium, only a limited amount may be added at one time. Some have complained that this qualifies this method as a "maintenance" calcium additive rather than an additive to increase calcium levels. Another disadvantage is that these products tend to be a little more expensive to use.

Tropic Marin:
I use the manufacturer's name to identify this method, because frankly I don't know the chemical formulation. This is a type of calcium additive that may be purchased as a powder. The appropriate dose is measured out, and simply sprinkled in the aquarium water. The obvious advantage of this method is its ease of use. Those who use it claim that it is quite effective at slowly increasing calcium levels, and maintaining them. The disadvantages include a gradual increase in salinity that must be watched and mitigated by the occasional addition of freshwater in substitution of aquarium water. Also, this method of calcium supplementation is reported to be relatively expensive, compared to other methods.

Polygluconate Calcium:
This calcium additive is typically purchased in the form of a liquid that is poured into the aquarium. The appropriate dose is determined based on aquarium gallonage, then poured into the aquarium. The most prominent manufacturer of this product is Seachem and is sold under the name Reef Calcium. The advantage of this method of addition is the ease of use. It can be measured and poured with speed and ease. It will not affect pH or alkalinity.

The disadvantages include reports that this method of calcium addition is best used for maintenance, not increasing calcium, as the amounts of calcium that can safely be introduced are relatively small. This type of calcium is "chelated"; meaning that the calcium is released in a time released formulation. Thus, typically other methods must be utilized to increase calcium levels. Additionally, due to its chelated formulation, special test kits must be used to measure the aquarium calcium levels. False readings will be obtained with typical calcium test kits. Finally, it has been anecdotally reported that the use of polygluconate calcium can cause problem algae blooms. The manufacturer denies this is a possibility, and many hobbyists report using the product without problem algae blooms.

Calcium Reactor:
A calcium reactor is a device that essentially dissolves calcium carbonate in a reactor chamber, then drips it into a reef aquarium. The reactor chamber is filled with calcium carbonate granules and sealed. Water is pumped into the chamber and recirculated through the media using a recirculating pump. Into the reactor chamber is also injected CO2, which lowers the pH of the recirculating water. At the lower pH (usually between 6.5 and 6.8), the calcium carbonate dissolves into it various components, one of which is calcium and another of which is carbonates. The drip from the reactor is then rich in calcium and rich in buffer. It is also reported that the reactor output contains other minor and trace elements into the aquarium, although assays of this seem to indicate these amounts are relatively unimportant.

The main advantage of this method is that it is the closest method to simulating what happens on the reef, namely calcium carbonate being recycled into bio-available components, including calcium. It also provides both calcium and buffer. A calcium reactor, once installed and initialized, will maintain calcium/alkalinity levels at very stable high rates, with minimal routine maintenance or adjustment. Finally, the ongoing maintenance costs of a calcium reactor are far lower than any other method of calcium supplementation, costing typically well under $100/year.

The biggest disadvantage of a calcium reactor is its initial cost. A typical setup including the reactor, CO2 equipment, and pH monitor will cost $500 to $700 and more, depending on the size of reactor needed. Typically, the lower cost quoted above is associated with calcium reactors that will maintain tanks up to 200 gallons. It is recommended that a pH monitor be used, but is not required. As with the Kalkreactor, there are many quality DIY sites on the Internet, and calcium reactors are relatively easy to construct given the ability to do some simple plumbing using PVC. This significantly reduces the purchase cost of a reactor.

Other reported disadvantages include phosphate increases, and CO2 leakage into the aquarium. The reports of phosphates being dissolved from the media back into the aquarium have created concern; given the knowledge that calcium carbonate contains some phosphate. However, in media designed to be used in calcium reactors, phosphate levels are very minimal and not a significant contributor to phosphate levels in the aquarium. CO2 "leakage" can be a problem in that excess CO2 in the reactor's water output can cause a decrease in aquarium pH. However, when a reactor is properly set up and adjusted, this is typically not a problem as the drip rate is slow enough that excess CO2 is released through normal gas exchange processes in the aquarium.

Calcium Testing
Before concluding this discussion on calcium, I want to mention calcium testing. It is highly recommended that hobbyist acquire quality test kits for pH, alkalinity and calcium. Without accurate information and readings on these levels, it becomes much more difficult to obtain the desired water quality parameters. And in the case of the polygluconate calcium additives, it is important to get the correct test kit for measuring chelated calcium levels. Finally, in the use of such test kits, it very important that the hobbyist take the time to clearly understand the instructions and procedures involved in the kit they've chosen. Many errors and frustration have occurred simply due to erroneous test readings.