The RSM 650 tank you have uses a gravity feed ATO system with a float valve. This is fine for an ATO but it's not the best way to douse kalk. First off, you will need to mix the kalk with the ATO water every time you fill the ATO. You can not mix kalk in bulk because once it's exposed to air it quickly starts to loose it's effectiveness. The other problem you will have is that your adding something to the water and it has the potential to clog the float valve.
If your going to douse kalk, a much better way is to use a kalk reactor. This is a sealed unit that can constantly mix kalk into the ATO water. However this usually requires that the water be pumped through the reactor. This would also mean that your going to need an ATO controller to work the pump.
If all this sounds like too much trouble, I would agree, which is why I don't bother with kalk anymore. I tend to like equipment that is "set it and forget it".
For your system, I'd recommend that you setup the tank, leaving space for a kalk reactor and a calcium reactor that could be added in the future. Note a kalk reactor and a calcium reactor are not the same thing.
Run your system and see how you make out. Depending on the corals and other livestock being kept, you may be able to keep up with calcium levels with only water changes or using a 2 part product. If you can't keep up with calcium, then consider a kalk reactor or a calcium reactor and all the other gear that goes with either reactor.
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