07-06-2008, 11:37 PM
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It would help if you could provide details, or photos of your system (Overflow, Sump, Return, and Return Outlet)
The proper solution will rely on the root of your problem. As stated above, a small hole in your return line just below the waters surface, called a "siphon break", will eliminate water from back siphoning through your return line. But if your sump does not have the extra room to accommodate the water that drains out of the display until the water level reaches the low level point, you will never stop the floods without redesigning the sump.
The height of the overflow has nothing to do with the flooding. it only affects the level of water in the display. As you raise the overflow, so do you raise the low level point.