I get this question all the time at the lfs I work at. There are no fish that eat fish crap, and if they do, they will produce crap anyway. The only way to have a clean tank is to clean it yourself.
OK, I have a heavily planted tank. I cannot use my python to siphon off the bottom. There are just too many plants and not enough open areas to do so effectively. I’ve been under the impression that the plants will take care of the mulm that builds up down there. I also have Malaysian Trumpet Snails doing their thing, digging around the substrate, as well as Sterba’s Cories. I do feed my fish live black worms, so maybe there are some of them down there too… who knows? I also do very regular water changes as it’s also a Discus tank.
it depends on tank size, stocking, amount of water changed, how often water changes are done, etc. do you test your water? what are your nitrate readings? if they get high (above 20 for discus, higher for other fish) the more likely it is that stuff is building up and affecting your fish. if you do large water changes often and keep nitrates low, there won't be any toxic buildup.
Shrimp and inverts will process any undigested food matter out of waste from your fishes. I've seen them go to town on my pleco's poop after she ate a hefty meal of steamed broccoli. There was nothing left when they were done.
But my bichir made short work of the 20 I originally bought... there's only one left and my tank bottom seems a LOT dirtier again. ^_^;;
My wife loves the little cories too! She says they remind her of curious puppies. Her favorite though, for some odd reason, is the Bristlenosed Pleco who hides in the shadows all day…
Also, thanks for the reassurance guys; I do my regular WCs, so Nitrates won’t be a problem.