Check out Melev's Reef - Sumps & Fuge's
The sump has 2 or more compartments separated
by baffles. If your tank is not pre-drilled you will need a HOB
overflow. Water from the tank flows into the overflow and down to the sump. The overflows are designed to break siphon when the tank water goes below a certain level. The returns also need to be situated so that the back siphon is broken before the sump overflows. My sump can hold about 30 - 35 gallons. It normally has about 8-12 gallons in it.