Any reef aquarium hobbyist that attempts a stony-coral dominated aquarium should be very familiar with the calcium and alkalinity levels of their saltwater. Let's take a look at the role of alkalinity in our reef aquariums.

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 speaking to chemical change which is highly desirable when trying to grow sensitive organisms like coral.

In addition to chemical stability, Alkalinity is important for stony coral growth because corals and other organisms use up alkalinity over time. It’s measured in “degree of carbonate hardness” or dkh for short and natural sea water has a dkh of around 8 or 9. Like with any chemical parameter, you never know if you need to be supplementing it if you don’t test for it.

A Typical Alkalinity Test

The Salifert alkalinity test (Amazon Affiliate Link: is a basic titration which is a test of buffering capacity. It is made up of two reagents. The first is a bluish stain. The second reagent is what we will be adding drop by drop to perform the titration. The idea is once the buffering capacity of the sample is exceeded, the blue stain will quickly change to a pink color. The amount of that second reagent needed to change the color from blue to pink will determine the dkh of the saltwater sample.

The second reagent is a clear liquid that we will draw up using our 1ml syringe. The key here is to draw the plunger back until it lines up with the 1.0 mark. Again, don’t keep pulling back until the water in the syringe lines up with the 1.0 mark. That is going to give you a very different reading. Usually when the plunger lines up with the 1.0 mark, the fluid lines up with the 8.5 mark or close to it. This is fine because the pink plastic tip holds about .2ml bringing the total to the desired 1.0 ml.

When doing the titration, add the reagent drop by drop until you see a color change. We want to see the exact amount of reagent it takes. A good titration will show an immediate change once the buffering capacity is exceeded.

Now to determine what the alkalinity level is, take a look at the syringe and see that the end of the stopper is now at the .55 ml mark. So we’ve put in .45 ml into the sample before the color change. Referring to the instructions, we can see that a reading of .55 means the dkh reading is somewhere between 6.7 and 7.0. This is a little low considering natural sea water is a little over 8.0.

Adjusting Alkalinity

Before going off and trying to raise the alkalinity levels, consider the appearance of your corals. If they are doing wonderfully, it may be better to simply maintain the current levels rather than attempting to change them. Also consider that these test kits are not perfect, and also sometimes your technique is not perfect. A worst case scenario is the water chemistry is fine and a poorly conducted test encourages you to fix something that wasn’t broken in the first place.

Having said that, there are several ways to boost calcium and alkalinity such as additional water changes, kalkwasser, two-part additives, and devices such as calcium reactors. If those solutions aren’t quite working out, there is one other thing to look at which is the Magnesium level. Magnesium can affect the balance of Calcium and Alkalinity and that will be the subject of part III in our chemistry series.

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. If you would like to support this blog, please take a look at the test kits used by clicking on this Amazon Affiliate Link:

Than Thein