I personally do not like the idea of adding kalk to topoff water. IMHO, they should be kept as separate functions. Here's why: topoff water consumption is based on the tank's evaporation rate. Evaporation rates vary - especially with the wild changes in humidity we get here in San Diego; some days/weeks a tank might need significantly more topoff water than others. So, if you add kalk to your ATO, the dosage of kalk will also fluctuate. And this is contary to one of the prime tennets of reefkeeping... stability and consistentcy. When I was doing kalk, I used a kalkwasser reactor; supersaturated kalkwasser was dosed by an AquaLifter, with pumping timed by my AquaController. Over a period of several weeks, I determined the best amount of kalk to dose on a daily basis, and split that up into multiple small dosing periods per day. The kalk reactor was supplied pure water from my ATO resevoir. While this did replace some of the water lost due to evaporation, my ATO unit handles all additional makeup water needs.