If your top off system is gravity fed to a float valve the kalk will constantly plug the float valve so that is not recommended. If your ATO is administered by a pump the kalk will constantly plug up the pump so that is also not a good idea. What you want to do is administer kalk in line with an ATO system incorporating a pump (like the Tunze or JBJ systems) through some sort of reactor or perhaps a simple container. A reactor like the one Precision Marine makes can be purchased or built DIY if you're a little handy. Or a simple container like what Tunze sells could be made out of an empty pop bottle. These inline systems also have the advantage of keeping your top up reservoir clean and limiting the exposure of the kalk solution to CO2.
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