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Author:   Robert M. Metelsky
Title: What is Kalkwasser? And how do I dose it?
Summary:  Kalkwasser/limewater/calcium hydroxide should be used for water added to make up for evaporation losses.  It must be administered with care.

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Date first published: 2000
Reprinted from Aquarticles:
January 2004: The Sea Star, Wasatch Marine Aquarium Society
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Robert M. Metelsky,
Shoreline Resources,
POB 954,
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#205 - 5525 West Boulevard
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What is Kalkwasser? And how do I dose it?

By Robert M. Metelsky
Author of the book Simplified Reefkeeping, 3rd ed. available at


Because the tank is exposed to air, and strong pumps are circulating the water throughout the filtering system (overflow pipes, drip plate, prefilter, and protein skimmer), you will get a significant amount of evaporation. In fact, the better your skimmer and the stronger your water pump (both desirable features), the more evaporation you will get. You will need to replace the evaporated water regularly. An important reminder for the new hobbyist is that the water evaporates, not the salt. Do not add salt mix with the make-up water. The result will be a higher salinity than is desirable.

Adding make-up water provides a good opportunity to replenish much-needed calcium, which gets depleted rapidly in an enclosed reef system. This vital element is used by virtually all living creatures. Some of it also gets removed by protein skimming. In my opinion the best calcium additive is “Kalkwasser,” which is calcium hydroxide. It is added on a regular basis by mixing it with the purified water being added to compensate for evaporation. These regular additions of calcium hydroxide also keep the pH elevated to the desired 8.2 to 8.4 level.

Kalkwasser is a German word. Literally, it means “lime water.” Kalkwasser is a trade name for calcium hydroxide. The terms “Kalkwasser,” “limewater,” and “calcium hydroxide” all mean the same thing in this hobby.

The water you use to replace what has evaporated will be called “make-up water.” It is extremely important to use purified tap water mixed with calcium hydroxide (a.k.a. Kalkwasser, a.k.a. limewater) for the make-up water! Do not, I repeat, do not, use regular tap water or anything else for make-up water! This is asking for trouble.

As I have stated from the beginning, nothing will ensure your success more than the quality of your water. Once you have made the investment of a water purifying system and have started the reef with purified tap water, the reef will be accustomed to that quality of water. It would be extremely foolish to try to cut corners here. This is the last place to skimp. In fact, it would be inviting disaster by possibly introducing impurities (metals, silicates, phosphates, etc.) that are harmful and troublesome (hard to remove) into the pristine environment that we have tried so hard to create.

When to add make-up water:

Add the Kalkwasser within a day after you mix it; it gradually loses effectiveness after it is mixed Watch the water in the sump! This is where you will see the change in water level. Once you have established the “working water level” in the sump, mark it on the side of the sump box, with magic marker. This will give a quick visual reference as to the height of water that is normally in the system. As evaporation occurs, watch this mark. When the level goes down by 3 to 5 gallons, or gets close to the top of the outlet for the pump, you need to add make-up water. Mix the water no more than one day before you add it to the tank; it starts to lose effectiveness right after it has been mixed. It will have the highest concentration of available calcium just after the sediment settles out of the solution.

On a smaller tank (even a 55-gallon), 5 gallons of high-powered make-up water must be used with caution! Kalkwasser has an extremely high pH. Pay close attention to the drip/dosing flow of water, to be certain that it is administered very slowly. For a 55-gallon tank, you should take a 48-hour period to administer 5 gallons of make-up water. Make sure you test-run your drip method, to be sure that it introduces the desired amount of make-up water over the correct period of time. Adding Kalkwasser too fast will cause pH shock, which can be fatal or, at the least, unnecessarily stressful to the livestock. Take the recommended precautions and do not let this happen!

On larger tanks, 125 gallons and up, 5 gallons of make-up water will not have as much of an effect as it will in smaller tanks. For a 125-gallon tank, the Kalkwasser can be added at the rate of approximately 5 gallons in 8 to 12 hours. In a 200-gallon or larger tank, the 5 gallons can be added without any clamping system, allowing the airline tube to empty the 5-gallon bucket unrestricted. This will take less than 1 hour.

Time of day to add:

Another suggestion is to add the Kalkwasser mix when the tank lights go out, or (ideally) first thing in the morning. While the lights are off, the pH drops, reaching its lowest level the next day just before the lights come back on. If you add the Kalkwasser during this reef “night,” the effect of raising the pH will not be as significant as it would be during lighted hours.

There may be some questions and concerns about adding 5 gallons of Kalkwasser all at once. Yes, some critics may be correct that adding smaller amounts more frequently would be a less risky, less stressful, and more natural approach. However, I have used my method on tanks from 55 to 200 gallons, with no adverse effects, and I have not lost one creature due to pH shock. You do have to be careful on smaller tanks, but once you get familiar with this system, I’m sure you will find it to be very practical: (1) you will add make-up water less frequently, and (2) on larger tanks (125 gallons and up), you can add 5 gallons of make-up water at a time, which is a significant, convenient, easily measurable amount of water to add.

Do not mix with an airstone; this will add carbon dioxide and oxygen, which will reduce the effectiveness of the calcium hydroxide and defeat its purpose!

Benefits of adding Kalkwasser:

You may be interested in why it is so important to add Kalkwasser. Some of the benefits are:

*   It adds calcium that is needed by most of the creatures in the reef.
*   It encourages the growth of pink and purple coralline algae.
*   It keeps the pH elevated. By adding Kalkwasser on a regular basis (make-up water) and doing water changes every 2 to 3 weeks, I have found my pH to be consistently between 8.2 and 8.4. Keeping  the pH at this level makes it less likely that micro-algae will become a problem.
*   The reef just seems to love Kalkwasser.
*   There are many more scientific and chemical reactions that are beneficial. Take my word for it: adding Kalkwasser on a regular basis is one of the most beneficial procedures for maintaining a healthy reef and desirable water chemistry.