How To Maintain Calcium Levels

Calcium is a building block on the reef and is essential. Many corals, invertebrates etc depend on it.

Let’s discuss parameters first, then look at how calcium can be provided.

It is generally accepted that in a home marine reef aquarium a calcium reading between 350ppm and 450ppm is acceptable. A calcium level in the aquarium which is higher than 550ppm will normally precipitate out of solution. This means the water becomes calcium saturated and no more can be held. Because of the water being saturated the calcium forms calcium carbonate. This is not a state of affairs that we realistically want.

If [tag-tec]calcium[/tag-tec] is not added to an aquarium for whatever reasons levels will fall as the animals use what is available. Within reason, having calcium levels at a low level is not a threat to the animals which depend upon it. These animals however will not thrive. For this reason (as with all water parameters) it is best to maintain a stable and adequate reading.

There are various ways to add calcium to the water. In this post I will attempt to cover the most common methods.

Water changes

This has to be the easiest and simplest method there is to replace calcium along with other trace elements. Using this method does depend on what you keep in the aquarium. If you keep a large number of calcium loving organisms, such as hard corals, then you may need to investigate other additional alternatives.

If you keep, for example, soft corals then this method may be acceptable. If you do decide to use this method, (remembering that water changes should be done in any case), then I would recommend that you use artificial salt which is ‘high’ in calcium (check the package description). Another point to remember is that if you use reverse osmosis water, obtain artificial salt which has been designed for use with reverse osmosis water.

To check the calcium levels of the replacement water simply test with a calcium test kit.


Kalkwasser, otherwise known as limewater, is actually calcium hydroxide. Kalkwasser is a very fine powder and is normally introduced to the aquarium with the top-up water. There are realistically two methods to add kalkwasser to the aquarium, these are by a ‘kalk reactor’ or by what is called the drip method.

The drip method is where the kalkwasser is mixed with some prepared top-up water. It is important when mixing kalkwasser that it be mixed slowly, the reason for this is that it is imperative that as little air as possible gets into the top-up water. If too much air gets into the water then the kalkwasser will turn into calcium carbonate. Once the top-up water is prepared it should be left to sit for 2-3 hours so that any sediment can settle to the bottom of the container. The mixture which is left above the sediment is what will be introduced to the aquarium. It is best to siphon this mixture out and dispose of the sediment. After the mixture has been siphoned out it is ready for use.

The [tag-tec]kalkwasser reactor[/tag-tec] is where kalkwasser is introduced into a sealed chamber, within this chamber is a stirring device which mixes the kalkwasser and water. Water is pumped into the reactor normally by the use of a dosing/peristaltic pump and this water, because of pressure forces water rich in kalkwasser into the aquarium.

Obviously the kalk reactor is easier than the manual method but both methods do work. Let’s move on to dosing methods.

It is important when dosing kalkwasser not to dose it quickly. The reason is that water mixed with kalkwasser is of a very high pH. Therefore introducing it too quickly can alter the pH level of the aquarium water. To get round this problem you could either use a pH monitor to control the kalk reactor, or drip the mixture into the aquarium at a rate of about 1 drip per second (always drip into a high flow area).

There are both advantages and disadvantage in using kalkwasser in an aquarium. The disadvantages are two fold. One is that if you do not use a kalk reactor it takes time to mix the solution, the other is that because of the kalkwasser being added with the top up water you may not be able to introduce enough to maintain a steady level of calcium. The advantage, though, is that kalkwasser is very rich in calcium and can, if used correctly, maintain a high level.

Calcium Additives

Calcium additives are used in aquariums which do not have very many calcium demanding life forms, or where a small tank is used. For example you probably would not be able to maintain an adequate calcium level in a heavily stocked SPS tank using this method – that is unless you are willing to spend a lot of money on lots of bottles!

The use of calcium additives is quite simple. Normally on the top of the bottle is a measuring device, often the lid itself, and you simply add a certain amount of liquid dependant upon the net gallons of water in the aquarium.

Calcium additives are useful alongside water changes. If you are not able to maintain an adequate level by performing water changes alone then you can add extra calcium using additives.

There is another type of calcium additive on the market which is not liquid but in powder form. This powder is mixed into some salt water and added to the aquarium in a high flow area, keeping away from corals etc.

Another calcium additive is called a balanced additive. This is a two part additive where you can add calcium with one and maintain alkalinity with the other.

Follow the manufacturer’s instructions on any additive. These will include a given amount of calcium and how much it will raise the level in ‘X’ gallons. Always make any increases slow, never adjust in one go.

Calcium Reactor

This is the method of choice for most reef keepers who keep calcium dependant organisms and/or have medium to large aquariums.

I am not saying that you definitely require one of these devices. It depends on what you are keeping. I would recommend that you try the water change and calcium additive method first. If you are struggling to maintain calcium levels in your aquarium, or the cost of additives is excessive, consider this method.

How does a [tag-tec]calcium reactor[/tag-tec] work?

A calcium reactor is a device which holds calcium carbonate (suitable media is commercially produced and easily obtained. It comes in granular form.). The media is slowly dissolved and releases calcium, and more, into the water.

The reactor is a sealed unit. Water is pumped slowly into the reactor, and is circulated within the reactor by a built-in pump. The calcium carbonate will not dissolve at the pH levels of the water in the aquarium, therefore carbon dioxide (CO2) is injected into the reactor. This CO2 lowers the pH within the reactor to levels of about 6.5 to 6.8 and the media slowly dissolves. The resulting mix can then be supplied slowly into the aquarium.

It should be noted that it depends upon the calcium media used as to what pH level the water needs to be for the media to dissolve. Also dependant upon the choice of media is what other elements will be supplied into the water.

The main advantage of this device is that it provides the easiest way to add calcium to the aquarium. Once a calcium reactor is installed and setup correctly, it can maintain high levels of calcium. Maintenance is simple. One check that must be made daily is the rate of injection of CO2. Excessive injection could allow excess acidity to affect the aquarium water (also dissolving the calcium media too quickly).

The initial cost of the calcium reactor, plus the CO2 bottle and control, has to be the biggest disadvantage. However, once the purchase has been made the running costs are minimal.

  1. I found this a very informitive article, especially the information about the Calcium Reactor.
    The only mild disagreement I would have is with the use of Kalkwasser. I know this is popular, however in my tests this the use of Kalkwasser casuses imbalances in alkalinity and chemistry in general. I perfer balanced buffers or bio available calcium such as Sea Chems.

  2. I don’t use kalkwasser. I prefer to use seperate additives to maintain calcium and alkalinity. At first it is a bother, as testing is very regular to obtain the dosages that will maintain the seawater levels. After this period, it is simple, with ‘standard’ dosages weekly. A test is required to confirm the situation from time to time. I’ve been doing this for years now with no problems (to date!)

  3. Hi Carlrs,

    I do have to agree that kalkwasser can cause inbalances, however, if used wisely and carefully it can be beneficial to the aquarium – in the right hands!

    Personally I have used a mixture – water changes, additives, calcium reactor etc. I have to admit that a calcium reactor is my method of choice. It can be a bit fiddly at the start getting the bubbles right but after that all is ok – especially if you install a probe.

    Its up to the aquarists really and what is best for both the aquarium and for them.

    As long as corners are not cut…..

  4. hi there

    I have this particular anemone, it looks as if itÅ› dead but suddenly it just moves somewhere else in the tank. Now I do not have many soft corals, only 3.

    I have increased the lighting, water changes etc. the other corals are behaving excellent but this particular anemone is like flatten out.

    Any comments?

  5. Hi ATDM,
    I assume you are talking about one of the larger types of anemone.
    You say the three corals you have are fine. This suggests the water quality is OK. You have increased lighting and I must assume lighting (intensity, spectrum) is OK.
    It is well known that anemones, particularly some types, will go for a walk if things are not to their satisfaction. This can very often be to do with water currents, they like good water movement. Try producing some chaotic, or another name random, water movement by adjusting powerheads. Don’t make the current over strong or laminar, it won’t like it. Don’t allow a powerhead to point straight at the anemone (or any other coral). If possible, put the anemone in the top third of the aquarium, for maximum light. Anemones like their base to be secure, so try and allow the base to sit in a hollow in the reef structure.
    If you are wondering if your calcium level (the topic of the blog) has anything to do with the problem, then no, this is very unlikely.
    Hope this helps. 🙂

  6. good post…………….

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