ca161406, You didn't mention if your tank was drilled or not. If it's drilled then in the event of a power outage the tank will drain down to the lowest 'hole' in your tank whether it's the overflow or the return. If your tank is not drilled, then yes the tank will drain down to the overflow box, but due to gravity pulling down the water in the return line water will be syphoned down until it drops below your return. I just set up a 30 Long as a sump on my 125 and the best way I found to stop the syphon on the return is to install a check valve. As soon as the flow up stops on the return the valve shuts and you get 0 water going back into your sump from the return. I am using an overflow box and basically the only water going to my sump when the electricity is shut off is the amount of water in the lines going to the sump, and it only raises my level about an inch or so.
indigoblutc, I am no expert on sumps, as a matter of fact I just built my first one, but I read a lot on here and other places about sumps and changed my design a few times before I settled the one I thought would be best for me. There is a debate on putting the skimmer in the fuge because some say that you are skimming water that inhabitants of your fuge need. I don't know if this is correct because it would seem to me that if your skimmer was before the fuge then that would be the same as if it was in the fuge. I know that you definately don't want it in the return area because you will be introducing bubbles in that chamber that might get returned to your DT. Some people say to put it in the input chamber because your bubble trap will take care of the bubbles produced by the skimmer. So I guess it depends on if you are building a sump big enough for 3 or 4 chambers. If you have the room I'd build one with input->fuge->skimmer->return. If not then go with input/skimmer->fuge->return or input/skimmer->return<-fuge.