Kalkwasser, otherwise known as limewater, is actually calcium hydroxide. Kalkwasser is a very fine powder and is normally introduced to the aquarium with the top-up water. There are realistically two methods to add kalkwasser to the aquarium, these are by a ‘kalk reactor/stirrer’ or by using what is called the drip method.
The drip method is where the kalkwasser is mixed with some prepared top-up water. It is important when mixing kalkwasser that it be mixed slowly, the reason for this is that it is imperative that as little air as possible gets into the top-up water. If too much air gets into the water then the kalkwasser will turn into calcium carbonate. Once the top-up water is prepared it should be left to sit for at least 2-3 hours so that any sediment can settle to the bottom of the container. The mixture which is left above the sediment is what will be introduced to the aquarium.
It is best to siphon this mixture out and dispose of the sediment. After the mixture has been siphoned out it is ready for use.
The kalk reactor/stirrer is where kalkwasser is introduced into a sealed chamber, within this chamber is a stirring device which mixes the kalkwasser and water. Water is pumped into the reactor normally by the use of a dosing/peristaltic pump and this water because of pressure forces water rich in kalkwasser into the aquarium.
Obviously the kalk reactor is easier than the manual method but both methods do work.
It is important when dosing kalkwasser not to dose it quickly. The reason is that water mixed with kalkwasser is of a very high pH. Therefore introducing it too quickly can alter the pH level of the aquarium water. To get round this problem you could either use a pH monitor to control the kalk reactor, or drip the mixture into the aquarium at a rate of about 1 drip per second (always drip into a high flow area). It is also worthwhile keeping an eye on the pH levels by testing the water using a pH water test kit.
There are both advantages and disadvantage in using kalkwasser in an aquarium. The disadvantages are twofold. One is that if you do not use a kalk reactor it takes time to mix the solution, the other is that because of the kalkwasser being added with the top up water you may not be able to introduce enough to maintain a steady level of calcium. The advantage, though, is that kalkwasser is very rich in calcium and can, if used correctly, maintain a high level.