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.

Given that I do not use kalkwasser, I have scoured my references and the web to put together the information required for you to use it safely and effectively. Let’s start.

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.

What happens if Kalkwasser is overdosed or dosed too quickly?

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.
Kalkwasser dosing is therefore a careful balance of providing enough to get the desired benefits, while not overdosing to the extent the pH level rises significantly.

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.
Additional lime can be added in small amounts to compensate for the calcium oxide or calcium hydroxide that gets converted into calcium carbonate by any dissolved carbon dioxide in the water.

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.
Allow the solution to settle for a few hours until the solution has cleared to a slightly milky appearance, and then decant the solution into a sealed storage or dosing container leaving behind any undissolved lime. This remaining lime sediment could be re-used for another batch, a lot of it will be relatively insoluble calcium carbonate and of no use, so you may as well dispose of it via the household drain.

How should Kalkwasser be handled?
Both the lime and the kalkwasser solution itself are highly caustic, i.e., they have a high pH.

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.
Open to air, the solution will lose its usefulness very quickly as the chemicals in the water react with the air, particularly atmospheric carbon dioxide. The amount of hydroxide in the kalkwasser goes down, and consequently its usefulness as a pH buffer diminishes.
Kalkwasser has been verified to remain chemically stable (and consequently useful) for between 2-3 weeks.

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:

1)    A dripper can be made or bought that includes a kalkwasser storage container and a controller for adjusting the drip rate. The drip rate is set to an absolute maximum of one-half liter per hour of saturated solution per 50 gallons of aquarium water (or the evaporation rate of the aquarium, if known). When the kalkwasser dripper is first installed, a much lower dosing rate is recommended until calcium and alkalinity depletion rates are known. If kalkwasser drip rate is less than evaporation rate, the dripper can be used alongside an automatic top-off (or ATO) system that adds the additional amount of deionized water from a separate reservoir. If the kalkwasser dripper needs to operate at a faster rate to provide the optimal water chemistry, this can be achieved using special controllers that use pH detectors and solenoids to start and stop dosing as required.
2)    An automatic top-off reservoir can be used to store kalkwasser solution that is added when a low water level is detected in either or the sump or the aquarium. However, the use of power heads or other centrifugal pumps can add kalkwasser at a faster rate than it required (or safe). For this reason, peristaltic dosing pumps are better suited to this type of system, because of the much lower flow rates such pumps provide.
3)    Commercially-available devices called kalkreactors and kalkstirrers can be added to the automatic top-off system. These devices maintain a surplus of lime above the saturation limit that is periodically stirred using a small motor. Water from the automatic top-off system carries the lime-rich solution from the kalkreactor into the aquarium when a low water level is detected in the sump or the aquarium. The deionized water added to the kalkreactor dilute the concentration of lime in the reservoir, and combined with the physical mixing inside the system, some of the surplus lime quickly dissolves into the water, producing a new saturated batch of lime-rich water ready for use next time the automatic top-off system comes into use. As before, a peristaltic dosing pump will provide the slow rate of flow required for dosing kalkwasser.

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:

1)    Dose kalkwasser at night when carbon dioxide levels are typically higher and pH is lower.
2)    Dose carbon dioxide to the aquarium. Dosing carbon dioxide with a carbon dioxide reactor and properly functioning and calibrated pH controller and solenoid valve can ensure that a prudent pH level is not exceeded while dosing kalkwasser.
3)    Use vinegar alongside kalkwasser. Distilled white vinegar can be used in two ways to help control pH. Distilled white vinegar is a safe carbon source ideal for this type of application. It is also economical and easy to obtain. Acetic acid in vinegar quickly breaks down into acetate when added to the aquarium; this is used by certain bacteria as a food source and they produce carbon dioxide as a waste product. The microbial processes involved bear some similarities to de-nitrification, and dosing with white vinegar appears to accomplish a certain amount of de-nitrification as well.

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.
When vinegar to the kalkwasser solution directly, from 3 to 12 ml of vinegar can be mixed per liter of saturated kalkwasser solution. Adding acetic acid this way also has the further benefit of allowing close to one-third more lime to the water than otherwise. It is recommended to start with a dose of 3 ml per liter of solution, and then observe your aquarium to see if there are any negative side effects (though my research hasn’t pinpointed any specific side effects to look out for).

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!

References
A Simplified Guide to the Relationship Between Calcium, Alkalinity, Magnesium and pH. Randy Holmes-Farley in Reefkeeping Magazine
What Your Grandmother Never Told You About Lime. Randy Holmes-Farley in Reefkeeping Magazine
Water Quality Guidelines for Marine Aquariums. Bob Goemans, ISBN 0966454987, Marc Weiss Companies (2002)
The Reef Aquarium: Science, Art, and Technology, Vol. 3. J. Charles Delbeek & Julian Sprung, ISBN 1883693144, Two Little Fishies (2005)