Any reef aquarium hobbyist that attempts a stony-coral dominated aquarium should be very familiar with the calcium and alkalinity levels of their saltwater. Calcium and Alkalinity are vitally important chemical parameters in reef aquariums because they are integral to stony corals building their calcium carbonate skeletons.

What is Calcium?

Calcium is one of the major ions in saltwater. Calcium is the 5th most common ion in saltwater behind, chloride, sodium, sulfate, and magnesium. In most healthy reefs, the calcium level hovers between 400 to 450 parts per million (ppm). Reef aquarists are typically more keenly aware of their calcium level than any other parameter. It enjoys a degree of familiarity that other ions do not.

What is Alkalinity?

Alkalinity is a bit more abstract than calcium. It is not a particular ion, but rather the interaction of several that affect the buffering capacity of saltwater. Buffering capacity can be thought of as the amount of acid required to lower the pH of saltwater to the point bicarbonate turns into carbonic acid. It sounds over technical, but in layman’s terms, higher alkalinity levels equate to greater chemical stability in our reef tanks. The more acid required to cause that chemical change, the more resistant the solution is generally to chemical change which is highly desirable when trying to grow sensitive organisms like coral.

Maintaining Calcium and Alkalinity is not Straightforward

Corals and other organisms that build skeletons or structures out of calcium can deplete the levels of calcium and alkalinity over time. The whole point of having healthy levels of these two major ions is so that it is bio-available to the inhabitants of our tank. On quick glance, it would seem that simply adding the desired ingredient could easily solve the problem. For example, if your reef tank had a calcium level of 300ppm when you desire a value closer to 400ppm, you could theoretically add a calcium supplement to boost it. Unfortunately, reef aquarium chemistry is rarely that straightforward in practice. Addition of a calcium supplement in this manner often comes with a corresponding fall in alkalinity levels.

This problematic seesaw effect between calcium and alkalinity stems from how the two ions interact with one another. The two ions combine to form calcium carbonate and fall out of solution, thus lowering both levels.

Here are five practical techniques for maintaining healthy levels of calcium and alkalinity.

Option #1: Water Changes
The first technique is very basic. Water Changes. Water changes help manage fluctuations to a large degree. Most salt mixes available in the hobby today are formulated to have slightly higher concentrations of both calcium and alkalinity. Frequent water changes replenish major elements as well as trace elements.

For reef systems that are packed with stony corals, additional supplementation may be required. That brings us to…

Option 2: Kalkwasser
Kalkwasser is an age-old supplement that is highly effective in boosting both calcium and alkalinity. Kalkwasser is calcium hydroxide and is considered a balanced supplement that boosts both calcium and alkalinity together.

Kalkwasser comes in a white powder that you mix with purified water. It gets very cloudy for a few hours but after that the cloudy particles settled leaving a clear super saturated solution. That solution is the kalkwasser that is then added very slowly into the aquarium. It is important that this solution is added slowly because it is very concentrated and can precipitate out quickly thus defeating the purpose of adding it in the first place. You will know you are adding it too quickly when the water looks cloudy after kalkwasser addition.

When properly administered, kalkwasser works beautifully. Many hobbyists use kalkwasser as their top off water rather than using regular purified water.

Option 3: Calcium Reactors

A more automated method of maintaining calcium and alkalinity is a calcium reactor. These devices slowly dissolve calcium carbonate media in their reaction chambers and slowly introduce calcium and carbonate ions back into the tank. They are great for keeping water chemistry rock solid for months.

Option 4: Two-Part Solutions
As great as kalkwasser and calcium reactors are, they are balanced supplements, which are not particularly effective when there is an imbalance in the reef. If the value of one component is low but the other is normal, using a balanced supplement will not work.

The best option in this situation is a two-part additive. They come in separate bottles, one for calcium, and one for alkalinity. Two-part additives can be added in different amounts and over time the levels can be boosted. Like I mentioned before, it’s not particularly effective to add just the calcium portion or just the alkalinity portion, but you can inch them up by adding in both with a slightly heavier dose of the component you want to boost.

Option 5: Magnesium

The fifth and final method of dealing with calcium and alkalinity issues is to take a look at magnesium. It may seem counterintuitive that the solution to calcium and alkalinity imbalances is to elevate magnesium, but the three ions interact regularly.

Magnesium is very similar chemically to calcium. It can bind up carbonate ions thus increasing the overall alkalinity of the water. If you find that no amount of tweaking calcium and alkalinity directly is helping, you may want to make sure it is not your magnesium level that is in fact low.

In summary, calcium and alkalinity are two of the most important chemical parameters to monitor closely in your reef aquarium. We hope this article was helpful in understanding them a little better.

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