Dont buy one from a site. Go to a store. Cheaper, no shipping. A reef ready tank has a built in over flow that brings water down to the sump. THen in the sump you have a pump to bring water back to the tank. THe different pieces of glass in the sump make up the different chambers of the sump. The return chamber, the inlet where you use 3 pieces of glass as a bubble defuser, a refugium and a spot for you heater, and skimmer and anything else.