01-25-2011, 05:51 PM
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While bacteria does "produce" nitrate in a sense because bacteria breaks ammonia down into nitrite and then into nitrate, if I understand it correctly, it is the waste/detritus in the water that builds up in external filters that causes a rise in nitrates and makes these filters a problem, the same way gunk getting caught in bioballs can cause a wet/dry to be a "nitrate factory". Washing your filter's sponge is a good idea because it removes the waste/detritus that would only break down into more nitrate, but not because it kills off bacteria that facilitates the nitrogen cycle.
Many marine hobbyists recommend forgoing canister filters in favor of a prefilter and sump sock to catch larger debris, and a protein skimmer to remove waste from the water column.