A reef tank also has higher power (ie. hotter) lights & more flow. These contribute to the evaporation. I have glass tops now on all my tanks except the main reef and that helps a lot for reducing evaporation. Some of my tanks though are fw or fowlr.

Well the main tank has glass tops on it. The sump does not. Not going to buy an ATO. I'll look around and find a DIY way to get the job done.

I would not cheap out when it comes to an auto top off. I did and had it running right off my ro unit i flooded my house and lost a lot of stuff in my tank as i pumped about 100 gallons of freshwater into the tank while i was at work. I would recommend a tunze and have a reservoir to prevent anything like this happening.

evap
 

My 20 gallon loses 1.5" a day when its hot out. It is evaporating.

I have two systems.

My RO to the storage tank has a shut off of its own.

The storage tank to the tank has one too. My storage tank is 5 gallons. My corals and fish won't much like the drop in salinity that 5 gallons will cause, however, it won't kill everything.

I am looking to also get the RO shut off that BRS has on their site. If it sense's water (flooding) it will pinch the RO feed line shut. It also has an audible alarm.

Link to a thread with some ATO solutions, mostly DIY.

http://www.canreef.com/vbulletin/showthread.php?t=60320

I find that my sump level decreases because of my overflow get clogged. So the water level in my overflow rises. Or my water level in my display rises as the "fingers" on my overflow get clogged and doesn't flow as well, so when I clean it, the water level in my sump rises by a few inches. So you might want to check your overflow as well.

thanks all for the tips.