If you have an uncovered or poorly covered aquarium the vast majority of cats will kill the fish. I have multiple aquariums, a cat, and for a few years I lived with someone with 2 more cats that definitely wanted to eat my fish. Most problems with aquariums are easily solved. Cat kills fish? Get a tightly fitting lid. It's safer for the fish anyway so they can't jump out, it reduces evaporation, and it keeps the air humid for anabantoids. Cat drinks out of your filter? Get the cat a fountain.