I have a 55g reef system. I have an old 21 gallon tank that i would like to use as sump. i was wondering if I could put my old filters on it, i could also place my prizim skimmer and my heater on it (so i dont have to look at them in the tank). i was thinking i could use pumps some how to get the water to and from the tank. This 21 is a normal tank with no over flows. I dont understand how a sump works with out overflowing. I would be willing to do simple motifications to the 21 gallon. Is this possible? This would somthing like a refugium with my old filters and skimmers running on it, only it is below the tank like a sump. Please help.