What is Kalkwasser?
Kalkwasser is a saturated solution of calcium hydroxide in fresh water. It can be slowly dripped into an area of high circulation to replace evaporated water. It can help to maintain both calcium and alkalinity. Kalkwasser also goes by the name "limewater".
Mixing Kalkwasser
Kalkwasser should be made by mixing calcium hydroxide in fresh water. Aged tap water can be used, but RO water will be better.
A saturated solution means that no more calcium hydroxide can be dissolved. Adding more calcium hydroxide than necessary ensures the solution is saturated so sufficient calcium hydroxide should be added to the water so there is some precipitate at the bottom. A heaped teaspoon of calcium hydroxide in 2L of fresh water is a good starting point and this can be adjusted so that you get some precipitate, but not too much.
Kalkwasser should be mixed at least one hour before it is to be dosed to the tank. This allows time for any small particles of undissolved calcium hydroxide and calcium carbonate to settle so that only the clear solution will be dosed.
Mixed Kalkwasser should be stored in a sealed container until it is ready to be dosed. This is to reduce any dissolution of carbon dioxide which can react with the hydroxide ions to form bicarbonate which will precipitate calcium carbonate.
Dosing Kalkwasser
Kalkwasser needs to be added slowly to the tank and to an area with a lot of circulation. Dripping slowly ensures that the pH of the tank water is not raised too high or too quickly. Dripping into an area of high circulation ensures the Kalkwasser is mixed thoroughly and doesn't cause a local high concentration of calcium which may result in precipitation of calcium carbonate. Kalkwasser can be dosed at any time in the day, however, dosing at night helps to counteract the typical lowering of pH that occurs in reef tanks at night.
The simplest way of dosing Kalkwasser is to use a soft drink bottle, airline and a airline tap or clamp. The Kalkwasser can be mixed in the soft drink bottle and the bottle should be capped and the solution allowed to settle as discussed above. At time of dosing, the bottle can be placed higher than the delivery point and the airline used to siphon the solution from the bottle to the tank water. The tap or clamp is used to slow the flow to a drip, around one drip per second.
The inlet of the airline should be pushed into the bottle to a depth of around 2 cm from the bottom of the bottle to reduce any precipitate getting sucked up into the tube. Not only will the precipitate cause blockages in the tube, it can cause problems if it makes it into the tank. If calcium carbonate precipitate makes it into the tank, it can cause further precipitation of calcium carbonate from the water, reducing both the calcium and alkalinity levels in the tank. If builder's lime is being used, the precipitate may also contain impurities which may dissolve in the lower pH of the tank water.
How often should you dose?
The frequency of dosing will be determined by the evaporation rate of the tank. If the evaporation rate is 1 L per day, 1 L of Kalkwasser can be added per day. If the evaporation rate is higher, more can be evaporated each day. If the evaporation rate is lower, Kalkwasser can only be dosed less frequently. Kalkwasser may not be sufficient on tanks with low evaporation rates and high calcification rates - see the section on calculations.
Sources for calcium hydroxide
Calcium hydroxide can be purchased as "Kalkwasser" made by aquarium product manufacturers such as Seachem and Kent Marine and available in aquarium stores or from online outlets. Builder's Lime is a much cheaper alternative and is available from most hardware stores and building suppliers. When using builder's lime, extra lime should be added to the water as the builder's lime will contain more impurities. It is important that the solution really is saturated as this decreases the solubility of most of the impurities.
Kalkwasser calculations
Correctly mixed Kalkwasser should have a calcium concentration of 900 mg/L (Fosså and Nilsen, 1996). With this knowledge, and given regular dosing of Kalkwasser, it is possible to calculate the approximate amount of calcium that will be added to the tank.
For example, for a 180 L tank with an evaporation rate of 2L per day, it will be possible to dose 2L of Kalkwasser per day. 2L of Kalkwasser should contain 1.8 g (1800 mg) of calcium. Therefore, 10 mg of calcium will be added each day for every litre of water in the tank. i.e. the calcium concentration will rise by 10 mg/L per day, before calcification removes it.
To determine if you are adding enough (or too much) Kalkwasser, take measurements of the calcium and alkalinity at the exact same time on consecutive days. If the levels are the same each day and within acceptable ranges (350-500 mg/L for calcium and 3.0-5.0 meq/L for alkalinity), you are dosing sufficient amounts of Kalkwasser. If the levels are increasing, you may be dosing too much and as the levels approach the upper levels of the acceptable ranges you should cut back on dosage. If the levels are dropping, you are not dosing enough. If you are already replacing all the evaporated water with Kalkwasser, you may have to switch to another method of maintaining calcium and alkalinity.
Vinegar - extending Kalkwasser use
It is possible to add small amounts of vinegar to the Kalkwasser solution to extend its capabilities. The vinegar works in two ways:
For more information on the use of vinegar in Kalkwasser, see
Expanding the Limits of Limewater: Adding Organic Carbon Sources by Dr Craig Bingman.
Kalkwasser warnings
Calcium hydroxide is caustic and should be handled with care. Avoid breathing the dust or getting it in your eyes.
Kalkwasser has a pH of more than 12 and should also be handled with care. Avoid getting on your skin or in your eyes. As the pH is high, care must be taken when adding it to the tank.
Kalkwasser Trivia
The term "Kalkwasser" was coined by Peter Wilkins and literally means "lime water" in German.
Nouns in German have their first letter capitalised and so "Kalkwasser", being a German noun, should have its first letter capitalised.
More information about Kalkwasser
Limits To Limewater...Revisited by Dr Craig Bingman.
The Degredation of Limewater in Air by Randy Holmes-Farley.
What Your Grandmother Never Told You About Lime by Randy Holmes-Farley.
Metals In Limewater by Randy Holmes-Farley.
References
Fosså S.A. and Nilsen A.J. 1996. The Modern Coral Reef Aquarium, Volume 1. Birgit Schmettkamp Verlag, Bornheim, Germany. 367pp.
Last updated: January 21, 2005