Mark,
I have an ACII on my tank. EVERYTHING it controls has backups - and I'd STRONGLY recommend that. In my experience, these things just aren't reliable enough to depend upon SOLEY. Others here just use them to control lighting - and I'd concur. I also use mine for ozone, but with my ozone unit, it's really not big enough to hurt the tank if it just ran. My chiller runs on it's own, with the ACII as a backup (to shut it down). I have fans that kick on if the chiller fails, too. And, a temp alarm on top of everything else.
What it comes down to is this. These units WILL fail. Just make sure your tank doesn't crash if it does.
P.S. We'll be at the boat all weekend in the marina. If you aren't working on Friday night - or any time - stop by for a drink. 210.663.8079.
Bill
215g FOWLR... and anemones, GSP, gorgonians... carp, that isn't FO!
"I killed my first SW Fish in 1971..."