That's just about right....but losing the flow through the U-tube is a very unlikely event. Most likely, this would only happen if an errant Turbo Snail got into the internal part of the overflow and blocked the flow through the U-tube. So it's a good idea to put some kind of cover on the top of the part of the overflow that's in the tank. That would make a problem of that kind even more unlikely. Still, if you ever do lose the "siphon," then the water wouldn't be making it's way down to the sump and once the water level in the sump goes down to the sump-return pump, the flow would stop. Depending on the water level in the sump, that's how much would be sent back to the tank without water being drained down. The flooding that would result would be the amount of water that would be pumped to the fishtank after your tank fills up to the brim and starts to overflow its sides and the sump-return pump running dry. But remember, this is like a worst case scenario because, again, the failure of an overflow with a U-tube is inlikely. By the way, you can ensure that this is never a problem by having two overflows if the tank's size would make it possible.