I have a whisper filter system for a 10 gallon tank. Well it seems the water flow is starting to slow down on full throttle, also the level seem to be almost at the level of the trickle knob. I tried to taking the cartridge out and washing it and that did help a lil bit but not much. Do i get another cartridge or replace the stuff inside the cartridge? I also read on here that i could cut a piece of sponge to fit the filter slot. HELP