laying a clean sponge on the top of the water every few days and letting it float around for a while will also take care of the problem without resorting to buying anything or replumbing your filter system. One of my issues with the serface skimmers is that almost all filter pumps operate a LOT better when the pick up intake is well below the serface of the water. This is due to the increased water pressue as depth increases. You can notice this effect during water changes when the level is lower if you watch the filter output.