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...
...but you knew that already...