Dosing Kalc

[Treebeard]

I am considering dosing kalc to my 90 gallon system. My current configuration is as follows:

My RO/DI water feeds a 4 or 5 gallon plastic tub on a shelf over my sump. I have a 1/4 line running from an air valve, mounted about 1" from the bottom of the tub, which gravity feeds a float valve in the sump to keep a constant water level.

What is the simplest method of dosing Kalc to this setup? Shall I mix kalc and add it directly to the fresh water tub? If I did, I would be constantly replacing evaporated water with kalc water. Would that be over dosing?

I considered picking up a TLF kalc reactor and running it directly into the sump, but I need to maintain a constant level in the sump for my skimmer which would be difficult to do with the kalc reactor and my ATO system. Alternatively, I could hook up the reactor to an alternate fresh water source and dose into the 4 gallon ATO tub periodically....

Any suggestions and tips would be greatly appreciated!