I cannot answer the kalk part. BUT I can answer to the ATO itself. I just bought it. It has been on the tank 2 days. I've tested it different times for different scenarios except a constant running pump scenario if the unit fails. I'm hoping there is a power cutoff that that doesn't happen. I'm sure there is, but stranger things happen daily. I use a return pump from a 29 biocube. I don't remember the gph, but I use a valve to restrict flow. Any pump can be used as long it is 50v or less, if I recall correctly. It has to run a minimum of 25 seconds and at most 10 minutes. The ATO delivers water with the valve around 1 minute and something seconds for where I have it set to fill.
The biggest issue I have with the device itself is that it takes a few seconds to register the overfill mark. Where water is concerned, seconds count. I have the feed line from my fresh h2o container fixed so that no siphon occurs. That is probably the most important issue to make sure doesn't happen.
I guess all in all, I like it. It does what it is supposed to and I can't ask it to do more.
