Yeah but the waste they produce is incredibly small. They probably remove more waste produced by a piece of fish poo than they produce on their own. While they don't eat all of the contents of fish poo they do sort through it and remove bits of undigested food that would otherwise rot with the rest of the poo. The sorting process also breaks up the feces and makes it easier for the filter to snatch up. Whatever isn't sucked up then finds its way down to beneath the substrate level that much quicker, making the nutrients more available to plant roots. That means the plants will lock up the decaying feces before it really has a chance to affect water chemistry.