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Hydrated lime bag 01.jpg
Kalkwasser is a saturated solution of calcium hydroxide (hydrated lime).


Kalkwasser is a German word and is used by marine aquarists to denote a solution of calcium hydroxide (or, occasionally, calcium oxide). The terms “kalkwasser”, “lime water” and "calcareous water" are interchangeable. Kalkwasser is often abbreviated to "kalk".

Kalkwasser is an economical and effective way to maintain calcium and alkalinity levels in a marine aquarium. Calcium is used in reef tanks by calcifying organisms, and is depleted over time. In tanks with many such organisms, water changes are unlikely to maintain high enough calcium levels and supplementation is therefore required, kalkwasser is one such method. In tanks with moderate to high calcium demands, kalkwasser may need to be supplemented with a calcium reactor.

Kalkwasser has been widely used in reef aquariums since the mid 1970s.[1]. There are three main advantages to kalkwasser over other methods of calcium supplementation:

  1. does not leave any toxic or harmful residue in the aquarium,
  2. precipitates phosphate from the water, and
  3. balanced supplement and will maintain alkalinity levels.

It has also been suggested that the hydroxide ions released from kalkwasser will react with heavy metals, such as copper, and remove them from solution if they are present in the aquarium.[2]

Preparing Kalkwasser

Safety Precautions

Calcium hydroxide and calcium oxide are corrosive and can cause skin burns. When working with dry calcium hydroxide or calcium oxide a dust mask should be used to minimise exposure to chemical dust. Gloves and safety glasses should be worn while working with these chemicals. These chemicals should also be kept away from children.

Dry calcium hydroxide and calcium oxide will react with carbon dioxide (CO2) to form calcium carbonate and must therefore be stored in an air-tight container to maintain purity.

Calcium Hydroxide or Calcium Oxide?

Most kalkwasser is made using calcium hydroxide, although calcium oxide can also be used. There is no relevant chemical difference. There are two reasons why calcium hydroxide is typically the better choice however. First, it is more soluble than calcium oxide and requires less physical effort to mix. Second, calcium oxide is slightly exothermic when it dissolves in water.

Mixing Kalkwasser

Kalkwasser is made by mixing calcium hydroxide or calcium oxide powder with water. It is preferable that pure water is used (RO, DI or RO/DI). Calcium hydroxide and calcium oxide are not very soluble, with only 1 - 1.5g of either dissolving in 1 litre of water with vigorous shaking. Unless a kalkwasser reactor is being used, the saturated lime water is decanted from the undissolved calcium hydroxide and added to the aquarium.

It is difficult to mix kalkwasser mechanically. The only reliable way to mix kalkwasser mechanically is by using a rotating articulated "stirrer" attached to an overhead motor, or by using a magnetic stirring plate. A powerhead cannot be used to mix kalkwasser reliably, as it will very quickly become jammed by calcium deposits. An air pump should never be used to mix kalkwasser as the injection of carbon dioxide (CO2) into the mixture will cause the calcium to precipitate as insoluble calcium carbonate.

Once made, kalkwasser must be stored in an airtight container, with as little air in the container as possible. This is because the kalkwasser will react with carbon dioxide in the atmosphere and form a "crust" on the surface of the liquid. This "crust" is actually calcium carbonate, which will eventually settle on the bottom of the container. If exposed to air, increasingly large amounts of calcium will be precipitated from the kalkwasser over time thereby lowering its potency.

Using Kalkwasser


Kalkwasser has a very high pH, around 12, so it will always be added slowly to an aquarium. If too much kalkwasser is added to the aquarium the high [pH] may kill the livestock. A good rule of thumb is that 0.8 grams of sodium hydroxide per 190 litres of water can be added to the aquarium every hour without raising the pH.[3]

There are a number of very simple ways that kalkwasser can be added to the system. A small siphon hose, made out of air line with a valve on the end, can be used to drip the solution into the aquarium. Another method is using a dosing pump to add small amounts at regular intervals. Sometimes the dosing pump will be used in conjunction with a float switch thereby acting as a top off. If a dosing pump is used, care must be taken to select a pump which is designed for use with kalkwasser and will not jam due to calcium deposits. Many popular commercial top off products, such as the Tunze Osmolator, are not designed for long-term immersion in kalkwasser.

If adding kalkwasser manually, it is better to add at night when there is a natural drop in pH. If kalkwasser is added using an automatic top off device, pH should be monitored closer, at least initially. This is to ensure that the addition of kalkwasser is not raising the pH.

Kalkwasser Reactors

One type of Nilsen Kalkwasser Reactor

The best way to add kalkwasser to the aquarium is using a kalkwasser reactor, sometimes called a Nilsen reactor. These reactors work by replacing evaporated water with kalkwasser. The Nilsen reactor uses a chamber in which a motor or a magnetic spinner keeps the kalkwasser agitated. Alternative designs use a small pump to keep the water circulating in the reactor and the solution saturated. The pump will ordinarily draw fluid from the top of the reactor, thus avoiding drawing precipitate into the pump impeller.

A float switch in the sump turns on a pump in a freshwater reservoir when the water level drops from evaporation. The top off water is pumped into the reactor. This causes the reactor to overflow into the sump. The main advantage of this design is that it eliminates the possibility that the dosing pump will be jammed by calcium deposits. It also a very low maintenance design, as the reactor can hold sufficient calcium hydroxide to last some weeks.

There are a number of commercial products available. For example, from Aqua-Medic and Deltec (using motorised stirrers), or from Reef Octopus (using a small pump). Alternatively, an aquarist can make a kalkwasser reactor for themselves, with many designs available on the internet.

Results from Kalkwasser

Regardless of whether kalkwasser is added as a slow drip, or as top off water to replace evaporated water, it will take several days if not more, to increase the alkalinity and calcium levels in an established reef tank. This is because only 1g of calcium hydroxide is being added per litre of top off water, while at the same time, organisms in the tank are removing calcium from solution. The overall net addition of calcium molecules can thus be quite small.

In this situation, some aquarists will use fans to increase evaporation, and thus increase the amount of kalkwasser added as top off water.

Overdosing Kalkwasser

A "snowstorm", caused by the precipitation of insoluble calcium carbonate.

Kalkwasser can be overdosed. It has a pH of 12, which itself will be harmful to the tank's inhabitants.

Additionally, at a high (but not lethal) pH, bicarbonate ions in the water will be converted to insoluble carbonates. In an overdosed system, kalkwasser has increased the level of calcium and carbonate ions in the aquarium to saturation. When combined with the high pH, this will bring about a precipitation event whereby carbonate precipitates into calcium carbonate, turning the water white. This is sometimes referred to as a "snowstorm".

If kalkwasser is accidently overdosed, the best remedy is an immediate water change. The pH will generally return to normal levels as gas exchange acidifies the water.




  1. (Delbeek Sprung 2005): Delbeek, J.C., Sprung, J., The reef aquarium: Science, Art, and Technology, Ricordea Publishing: Coconut Grove, 2005, p 210.
  2. (http://www.advancedaquarist.com/issues/feb2003/chem.htm): Holmes-Farley, R., Chemistry and the Aquarium: How to Select a calcium and alkalinity supplementation scheme, Advanced Aquarist's Online Magazine, 2(2), 2003.
  3. (Delbeek Sprung 2005): Delbeek, J.C., Sprung, J., The reef aquarium: Science, Art, and Technology, Ricordea Publishing: Coconut Grove, 2005, p 216.


What is kalkwasser?

Kalkwasser is a saturated solution of calcium hydroxide in freshwater (preferably RO, RO/DI or DI water). The solution is mixed and left to stand and the clear liquid is dosed slowly into the aquarium.

What would I use kalkwasser for?

Kalkwasser is used to help maintain calcium and alkalinity.

How do I know I need kalkwasser?

If you can't maintain calcium and alkalinity between water changes, you need to add both of these.

If you have organisms in your tank with calcium carbonate skeletons, such as corals, clams and snails, you will need to be able to replace the calcium and alkalinity they take from the water. Kalkwasser is one method to replace calcium and alkalinity.

How do I make Kalkwasser?

Mix 1-2 teaspoons of calcium hydroxide to each litre of freshwater (RO, RO/DI or DI) in a sealable container and mix well. For example, you can use a 2 litre soft drink bottle, add 2-4 teaspoons of calcium hydroxide and fill the bottle with freshwater. Put the lid on the bottle and give it a good shake. Allow it to settle before use.

How do I use kalkwasser?

Once the mixed kalkwasser has settled it can be added very slowly to the aquarium. Kalkwasser has a high pH, 12-13, and adding it quickly can cause large and potentially dangerous swings in the pH of water in the aquarium. As it has to be added very slowly, it is best to have some kind of drip system so that it takes a few hours to add a couple of litres. One simple method is just to use airline to siphon the water from the soft drink bottle and use a tap or clamp to regulate the flow.

See Dripping Kalkwasser.

How often should I dose kalkwasser?

If adding kalkwasser manually, every night if possible. The main purpose of kalkwasser is to maintain calcium and alkalinity. These will be at their highest levels straight after dosing and the lowest levels just before dosing again. To minimise the difference between the highs and lows and have a more stable environment, it is best to dose every night. If the load is low just dose a smaller amount each night. If the load is very low, dosing can be done every second or third night, but monitor the levels when starting this to ensure the variation isn't too high.

If adding kalkwasser through a reactor or other top-off device, it will be added to the aquarium 24x7. This is the best way to add kalkwasser, as it minimises swings in calcium an alkalinity. The pH should be monitored closely, at least initially.

Can I really use builders lime for mixing up to kalkwasser?

Yes. Builders lime is calcium hydroxide and has a satisfactory level of purity. It is less potent than a higher grade chemical because it contains higher amounts of insoluble calcium carbonate. More will thus be needed to produce a saturated solution.

What about plasterers lime?

Yes, it is predominantly made of calcium hydroxide just like builders lime. If anything, it will have a higher purity than builders lime, which can be seen by the fact that it may be whiter in appearance.

What about the impurities of builders lime, why won’t they poison my tank?

Any impurities in builder's lime are not very soluble at the high pH of kalkwasser and so will precipitate out of solution. It is advisable to use extra builder's lime when making kalkwassser to ensure the solution is saturated. Do not allow the precipitate in the container to be added to the tank.

Are the any dangers in using kalkwasser?

You can safely add kalkwasser to replace all water evaporated from the tank. As kalkwasser has a very high pH (greater than 12), it is essential that kalkwasser is added to the aquarium slowly and to an area with adequate mixing in order to prevent large swings in tank pH and precipitation of calcium carbonate.

When is the best time to dose kalkwasser?

The best time to dose kalkwasser is when the pH of the tank is at its lowest. This has two main benefits:

  • The high pH of kalkwasser can raise the pH of the tank. If the starting pH is lower, the pH will not be raised too high.
  • The pH of the tank tends to be lower due to higher levels of carbon dioxide. The carbon dioxide in the water combines with the kalkwasser to form bicarbonate which is the major component of alkalinity.

The pH of the tank tends to be at its lowest after the lights go out. When carbon dioxide dissolves in water it lowers the pH. The process of photosynthesis uses carbon dioxide and so when the lights are on and photosynthesis is occurring, there is less carbon dioxide in the water. This leads to a higher pH. When the lights are out and photosynthesis is not occurring, the pH tends to drop due to the increase in carbon dioxide.