Where do decomposers live in a fish tank?
Decomposers are microorganisms. They live in plant material in the fish tank,
No doubt they can live in a fish tank but it will certainly not do the fish any good. Turtles poo and piddle in the water and putrefy it making it deadly for fish to live in. They also bite chunks out of unwary fish. Please use your brains and don't try to mix Amphibians with fish. It can not and will not work in an enclosed aquarium.