We have a half-acre pond full of koi and comets, both of which are non-carnivorous species. We feed them catfish feed, on which they seem to do just fine.
A couple of years ago we gave about a dozen smaller fish to our daughter to put in her aquarium. She fed them tropical fish food. After about a year, the largest koi ate everything else in the aquarium!
If tropical fish food can turn peaceful, docile carp into killers, just imagine what it might do to cats!