The use of kalkwasser (lime water or calcium hydroxide) is often debated very strongly amongst its adherents and opponents. Although I do not use it, I have to admit that used properly, Kalkwasser is a safe, effective and economic supplement of both calcium and alkalinity in reef aquaria.
What is Kalkwasser?
Put simply, it’s the German translation of “lime water” or “chalk water”. You may have come across lime water for example in chemistry practical classes at high school. But for whatever reason, it’s the German name that’s become established in the marine fishkeeping hobby rather than the English names used in science and industry.
Kalkwasser is formed from a solution of fresh water and lime. Commercial kalkwasser powder is sold at a tropical fish shops, but there are alternatives. Many marine aquarists find pickling lime, calcium hydroxide, to be just as safe to use as commercial kalkwasser powder, but much less expensive. Another type of lime, quick lime, can also been widely used. Quick lime is calcium oxide, but when mixed with water a certain amount of heat is produced, so some caution is advised when using quick lime.
How does Kalkwasser work?
The complicated chemical reactions can be easily found in books and technical articles in fishkeeping magazines, but for the sake of this article, I will try to keep things as simple as possible. Advanced aquarists may consider what follows an oversimplification, but I won’t tell them if you don’t!
The kalkwasser solution contains water, calcium ions and hydroxide ions. The calcium ions obviously help to raise the concentration of calcium in the water, a good thing for animals and plants that use calcium for their skeletons. The hydroxide interacts with dissolved carbon dioxide to form bicarbonate, and together with any remaining hydroxide ions, this raises the alkalinity, thereby buffering pH and inhibiting acidification.
The interaction between the hydroxide ions and dissolved carbon dioxide is critically important. If there is too much hydroxide and not enough carbon dioxide, a reaction occurs that can reduce the amount of bicarbonate in the water. Excess hydroxide ions will raise the pH above acceptable limits.
To fix this, kalkwasser dosing needs to be stopped, and eventually the surplus hydroxide ions will be used up as carbon dioxide levels recover, for example through the production of carbon dioxide by fish and invertebrates.
How is kalkwasser prepared?
Find a glass or plastic mixing container with a tight lid, begin by adding the appropriate amount of fresh water, preferably reverse-osmosis or deionized water, though this isn’t essential. Add one-half teaspoon of lime per liter, put the lid on tightly, and shake vigorously for a few minutes to mix thoroughly.
Eventually you’ll reach the maximum amount of lime that can be dissolved in this quantity of water. This is called the saturated solution, and at 25 degrees C (77 degrees F) is at a calcium concentration of about 800 parts-per-million.
How should Kalkwasser be handled?
It is very important to keep these chemicals away from your skin, eyes, mouth and nose. Powdered lime is very fine and easily becomes airborne; consequently, when handling lime you should take sensible precautions including the use of safety glasses, rubber gloves and disposable face masks.
If you should spill lime or kalkwasser onto yourself, immediately flush the area with lots of water and then seek medical advice.
How should Kalkwasser be stored?
Kalkwasser solution should always be stored in an airtight container to minimize its exposure to atmospheric carbon dioxide.
How can Kalkwasser be safely dosed?
Kalkwasser is generally dosed as part of the water used to compensate for evaporation. The kalkwasser is added at a very slow rate though, otherwise the dissolved carbon dioxide will be depleted too quickly. There are several ways to do accomplish this:
What can be done to mitigate the high pH in the aquarium?
There are a few things that can be done to minimize the risk of high pH associated with dosing (especially overdosing) kalkwasser:
Vinegar can be added either separately or in the kalkwasser solution itself. When used separately, vinegar should be added to the aquarium alongside the kalkwasser dosing, most easily and accurately done via a dual channel pH controller and solenoids that dose each one as needed to maintain the pH within the required levels. Flow rates should be carefully set and maintained so that a malfunction here will not result in overdosing.
What if Kalkwasser dosing is not cutting it?
There are a few avenues to take if you find that even after adding kalkwasser, you aren’t able to maintain appropriate calcium and alkalinity levels. To start with, try increasing evaporation from the aquarium by using fans. As evaporation goes up, the addition of kalkwasser can be increased safely. You could also try increasing the saturation level of the kalkwasser solution by adding white vinegar, as discussed above.
Alternatively, you could use kalkwasser alongside other methods for raising and controlling calcium ion concentration and alkalinity. Two-part chemical solutions that balance calcium and alkalinity, or chelated calcium without alkalinity, are available. Note that calcium chloride should not be used though, because when used with kalkwasser, the result will be dangerously high chloride ion levels.
How do I establish an acceptable level of calcium and alkalinity prior to dosing kalkwasser?
Regular water changes using a good quality marine salt mix should bring all aquarium parameters closer to optimal. Two-part solutions can be used to raise both calcium and alkalinity to the required levels if necessary, assuming that they’re used at the rates recommended by the manufacturer. Chelated calcium products can be used to raise calcium levels without affecting alkalinity. Calcium chloride has been used to raise calcium levels without affecting alkalinity, but note the warning above about not using calcium chloride once kalkwasser is being used.
Calcium, alkalinity and magnesium levels should all be verified before employing kalkwasser as a method of pH control. Once kalkwasser has started to be used, regularly check these parameters as part of your normal maintenance routine.
Where do I start?
The first step is to get all the equipment required to dose kalkwasser at the correct amounts for your aquarium. If you are making your own kalkwasser rather than buying a ready-made mix, then you will need to obtain the necessary manufacturing and safety equipment as well. Disposable gloves and face masks can be picked up from drugstores inexpensively, while safety spectacles are available via biological supply companies, hardware stores, etc.
Test your water parameters to determine the correct levels for your system and from these the preferred dosing rates. To some degree this is trial and error: start off at the lower dosage levels, test the water daily, and make adjustments as required. Once you’ve established a dosing rate, keep testing periodically (at least weekly) to make sure your dosing rate works over the long term.
The addition of kalkwasser to the aquarium is a tricky but potentially very inexpensive way to optimize reef aquarium water chemistry. Although it isn’t without risks if done carelessly, as has been explained here, the basics aren’t difficult to grasp, and if you go slowly and keep an eye on your system, a kalkwasser system can help you enjoy years of happy reefing!