Kalkwasser has a very high PH.... pouring it into your tank will cause a PH spike, stressing or killing your tank inhabitants.
You have to administer kalkwasser by dosing or dripping it slowly... preferably at night so it will help to maintain your PH levels in your tank at a constant level (as PH drops at night).
For advanced aquarist who know what they are doing, they can mix the slurry into a highly turbulent area of their sump (anthony calfo method)... the risk is undissolved kalk particles burning livestock or pumps being choked/damaged due to kalk/calcium deposits buildup.
Do it the safe way... drip it.