most tanks have a box in one or both back corners, aprox 4x4 inch, tall as the tank, with a drain hole in bottom and slots in top to let water leave the tank, usually to the sump. Do a search on "overflow", "durso standpipe" and look for pictures.
Found a good pic on Rich Durso's site... Notice how the water flows from the tank into the "overflow box" and gets siphoned down into the sump via the standpipe...
