Hmmmm, sorry to barge in guys but I'm confused about what sort of overflow works without anything hanging off the back ?? Do they make a box that has two compartments that both sit inside the tank ??
I'm using a Lifereef system which has the black overflow box in the tank with another box hanging off the back. An acrylic tube runs from the tank overflow into the box hanging off the back, this is where the syphon comes from. Should the power go off the syphon stops when the water is lower than the box inside the tank. I drilled a hole in the return to break the syphon from that line. You obviously have to leave some room in your sump for this water should the power go off - or you shut it off to feed.
The problem isn't when the power goes off, its when it comes back on....no ??? If the power comes back on and you don't have a syphon coming from the tank back into the sump, you'll empty your sump via the return pump into the display overflowing it and maybe your fishes with it on the floor.
Does anyone else make an overflow that guarantees syphon start up when the power goes back on ?? I was under the impression that Lifereef was the only one, that's why I bought their product. Which by the way cost me 125.00 and its rated for up to 750 gph from your return pump. Mine measures 4.5 inches out from exterior tank glass but I think you could squeeze 4" out of it - but I guess that's not gonna work for you either. Bummmer
<center>
<a href="http://s602.photobucket.com/albums/tt109/whowadat/?action=view¤t=backside.jpg" target="_blank"><img src="http://i602.photobucket.com/albums/tt109/whowadat/backside.jpg" border="0" alt="Photobucket"></a><p></center>
I did put together a 20L for a sump and got a EuroReef C-2 skimmer for it barely used on RC, both for another 125 bucks. (250 total including Lifereef overflow) I couldn't be happier, I sure hope you find a solution!
Mike