Planted Tank VIP
Join Date: Jul 2002
Location: Victoria,British Columbia, Canada
The sponged do break down over time.. But it only seems to occur under a very high bio load. My 20 Gallon for instance, has never had a sponge replaced... But my 45 Gallon tank that houses several 3-5" fish (filtered with my Aquaclear 500) has had 2 sponges replaced in the last year and a half.
The sponges after a while get so clogged that the time between cleanings gets shorter and shorter... So replacing them once in a while is just a good maintenance practice. I also find that Cyanobacteria and other algae's can form in the sponge material itself and can hamper the water flow through them.
Its not a big deal when you consider the cost of the sponges is anywhere from $2-$10 depending on the size of the filter. Replace them twice a year and you're not looking at a huge investment at all.
I am actually a huge fan of these filters, and if I had the nerve to drain and move my 135 Gallon tank 2 inches forward I would add 2-4 Aquaclear 500's to it. I find these guys to be the easiest to maintain filter I have ever used. Simple design, simple but effective media, inexpensive media... You can't ask for much more.