I would like to add another skimmer to my system, I know have a CPR with a maxijet 1200. I perfer to add to the sump since there is enough things hanging off the back to the tank. But my sump is a converted wet/dry so it does not have but a few gallons of water in it at a time so how much/deep does the water have to be in the sump for the skimmer to work?