one thing u can do is buy a second sponge for it and have it in the system a week or two before u need to change it.. it wont pick up alot of the detrius and other things, since the water wont be forced thru it, but it will house alot of the bacterias... then when its time to wash the other, just put the 2nd one in its place, rinse the other really well, and put it back where the replacement one was.
i hope doesnt sound too confusing, LOL