The reason you mix Kalk with fresh water is because it needs a lower pH to dissolve, at a pH of 8.0 or more like we have in salt water it pretty much wont dissolve. The lower the pH of your water the more Kalk will dissolve in it, this is why a number of people will add small amounts of vinegar to their top off water, the lower pH means higher saturation of kalk. Super saturated kalk (meaning no more will dissolve in the amount of water you are adding) will have a pH of around 12, this is why people aim to drip it at night if you can control it. Our tanks go through a low pH phase in the late night after all lights have been out, everything that relies on photosynthesis will release carbon dioxide at night and this will cause our pH to drop. Adding Kalk at night helps to keep the pH from dropping to low, but this is also why we don't just dump kalk water in, the drastic change in pH could be so severe it could cause undue stress to our tank inhabitants.
Read up on kalk in the chemistry section.
Doug
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