A sump is a container holding water. Your fuge is basically a modified sump. I am going to run a 40g breeder tank for my 120g. It will have 3 compartments. 1st for skimmer, 2nd for return pump( basically a common sump compartment....water and a pump), with the last section being a skimmer. If you want I can email you a sketch. I got this idea from a reefer on this board although i cant think of their name