True. There is no fish that will eat fish waste or fish s%#t. However, if you want a fish to scavenge and clean up leftover food particles, then clown loaches and Corydoras are great depending on tank size. You may have to feed slightly more, as these fish shouldn't have to rely on scraps alone.
I've read that banjo cats will eat almost anything, including fish s@#t but they can't really be expected to eat only that. I've found that my banjos enjoy tubifex worms as well as flake food that falls into the substrate.
The metabolism of one teleost fish is enough like another's that you couldn't expect there to be enough oxidizable energy left over in fish feces to support a second fish. Second law of thermodynamics is having its effect. Check this out! www.secondlaw.com
Aren't you'all imagining a kind of organic pepetual motion machine? Like an economic system based on each of us taking in the others' laundry for a living.
An organism such as a snail, a detritus feeder that's much smaller in mass than a fish, and with both a lower level of kinetic energy and a higher efficiency in converting food to energy, can make part of its living converting unused carbohydrates in fish feces...