I have one asian needle nose gar, a black ghost knife fish, an angelfish, 4 rainbowfish, 1 paratilapia polleni, and a tiretrack eel. i put in 3 cubes of blood worms, the gar doesnt eat them only live fihs, so is that too much for the other fish.
there is no beneficial bacteria in the water itself. It rests inside of the filter media with some amounts on decorations and substrate. you could change all the water you'd like. Ammonia is being produced constantly by the fish. Not changing the water is just slowly killing the fish you have. Having more than 5 ppm of ammonia will make you grow the wrong kind of bacteria, as well.
This is spot on ^ ^
I agree 100% with everything dbanner has said in the thread. your gar might also not be eating normal market food because the water quality is stressing him out, aside from being a picky eater.