There's a good walkthrough for a DIY ATO using a float switch, aqualifter and relay here:
http://www.reefsanctuary.com/forums/...step-step.html
Wingedfish - I'm not doubting your setup, but I am curious. Are you putting the AC line voltage through the float switches? I have a number of concerns about that approach but the one that is easiest to convey is probably stray voltages.
Consider than when the switch is closed, the circuit is complete and current jumps between the two contact points. When the switch is open, the current will try to jump into your tank. Saltwater is a reasonable electrical conductor and the electricity will go looking for a ground in your tank. Have you noticed anything like this?
I suppose the hot wire is probably above the water line, but I think it would be difficult to completely isolate it.
Anyways, just curious.
- Brad