Sorry, didn't mean to confuse. No, you don't want nitrites and ammonia to be detectable. However, the more ammonia that breaks down to nitrites, the more nitrites that break down to nitrates. It's a matter of scale--the more waste that goes into the system, the more that comes out. So, the more waste there is from fish, the bigger the bacteria colonies become, and the most waste (ie, nitrates) the bacteria generate. This means if your reduce the waste available to the bacteria, they in turn will produce less nitrates. Removing the solid wastes before they break down is the handiest way to do this, other than removing the source, ie, the fish.
The prefilter will trap the solids where you can easily remove them. Just put a sponge on the intake to the filter--I use AC 22 sponge blocks, with a hole cut in one end. I do agree that the UGF maybe causing a problem--I've found that UGF will break large, easily removed solids into tiny, hard-to-remove pieces. When I quit using my UGF, all the sudden I noticed huge clumps of solid waste from my plecos.
