03-07-2010, 09:49 AM
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Flow rates tend to be slower when driven by airpumps, but a sponge can work fine as the only filter in the tank, depending on fish load and how heavily planted the tank is.
If driven by a powerhead, flow rates are higher, but the sponge tends to fill up faster, and requires cleaning more often. I've had powerheads collapse a sponge, due to the much higher suction as the sponge pores gradually clog.
Also, plecos like to eat sponges, so if used in a tank with a pleco, they will require regular replacement.