A portion of that answer comes from whether or not the fish of which we speak is a predator (carnivore) or not (herbivore/omnivore).

If a fish is normally designated as a predator, it stands to reason that that fish will grow faster on a diet high in protein.

If a plant eater (tilapia) then perhaps they don't need the protein..similar to cattle, which need some protein, but can get by on starches/carbohydrates.

In a lifetime, the average driver will honk 15,250 times. My wife figures I'm due to die any day now...