I lose about a g a day since I put up stronger fuge lighting and top up with tap water. I have an issue of sorts where my ph, alk and Ca levels a all really high but I don't get really worried as I am getting some coraline growth and the corals are all pretty happy. I'm not sure if that is from the top off water or not.

I looked at a few different ATO setups but those float switches everybody uses is less expensive ATOs seem to me like they could be trouble. I ordered a TOM Aqualifter which is a small air driver water pump that can move about 3.5 gph. I'm going to use it to pump make up water from a bucket and set it on a timer to control how much water it pumps. Should work pretty well I think and it only costs about $20.

2 years now without so much as a peep of trouble out of my cheapo ATO... (touch wood)... since I put a Kalkreactor instead of mixing batches of kalk I haven't had to descale the float valve... I've also kept my topoff limited to 7 gallons for when something DOES go wrong... I'm counting on it and 7 gallons won't make much difference in over 200 g of volume... It's all how they're thought out & designed. I had thought of a dosing system, read every where and decided to go this route & couldn't be more satisfied with my decision.