What is the purpose of an overflow box?

An overflow box is a great alternative to a drilled aquarium if you already have a tank and do not want to risk drilling a hole into it.

The overflow box will hang on the glass, using the water level to force water through the overflow box and into your sump. This is connected via pipes at the back or the side of your aquarium which lead from the outlet(s) on the overflow box, to the sump. The weir prevents fish from entering your overflow as well as creating a comb effect, allowing quiet, undisruptive flow.