"Kalkwasser" is the German word for "Lime Water". Since the Germans were the first to use this method of calcium addition I guess they got to name it. After all many of us follow a form similar to the "Berlin" method (Live rock, Live Sand, Skimmer) of reef keeping.
Kalkwasser is nothing more than Calcium Hydroxide. Some people may recognize that as "Pickling Lime" used to pickle vegetables. In fact some people do not bother getting high grades of Calcium Hydroxide, instead they go to the corner grocery shop and get Pickling Lime. It is a heck of a lot cheaper. However, who knows what is mixed in with this stuff. The impurities are visible to the naked eye.
For myself, I would rather pay a little more and get a higher grade kalkwasser mix. I picked ESV Brand Kalkwasser made by E.S.V. Company, Inc. This is an extremely pure product. According to ESV the product meets American Water Works and Water Chemicals Codex Specifications.
Kalkwasser is a cheap way of keeping your calcium levels high in your reef tank. There are different ways of using kalkwasser. I'll describe what I do. I add less than 1 tablespoon (1/2 to 3/4) to a gallon of chilled RO/DI (reverse osmosis/deionized) water. Tap water is rarely pure enough to be used in reef tanks. I mix the powder into the water until it is all dissolved. I usually do this in the evening and let it settle over night. Some people drip cloudy kalkwasser some people drip clear kalkwasser. Be aware that cloudy kalkwasser is very risky to use, I chose to drip the clear. This means that I let the kalkwasser settle out to the bottom of the container. I then siphon the clear liquid into to my kalkwasser dosser and drip that into the tank.
Fresh Kalkwasser has a pH around 12. It is something you must handle with care, don't breath in the powder it is toxic. If you make more kalkwasser than you can use in a day try to store it in a cold place like the refrigerator. The calcium will not precipitate out of solution as quickly. I still would not store it more than 48 hours. It's effectiveness is greatly reduced with time. If the pH of Kalkwasser drops below 11 (which happens in a few hours) its just about useless. Kalkwasser should be added to the tank slowly. The general method is to drip it into the tank or sump using some form of an IV bottle dripper, air valve, or just a knot tied on the line. Add the kalkwasser to an area of very high water flow to prevent localized precipitation of calcium.
To the left you can see the Kent Marine 5 Gallon Aqua Doser Kalkwasser dripper that I used to use when I added kalkwasser. I now use a Calcium Reactor to supplement my calcium needs. I rarely, needed to fill it to the 5 gallon mark buts nice to know I could do that if I need to. On the table is the IV dripper and flow rate controller (blue) which plugs into the left side of the doser.
The actual kalkwasser powder should be stored in a cool location, in an air tight container. You'll notice I keep the seal of my jug taped shut. Carbon dioxide in the air will react with the calcium hydroxide and make calcium carbonate -- which will not dissolve in water easily. This is another reason why kalkwasser water should be used quickly. The longer it sits around, the more time it has to react with the air and produce useless calcium carbonate.