You should be feeding foods tailored for fish. Cat food is going to contain a lot of terrestrial meats, which aren’t healthy for fish due to higher fat content (plus other chemicals that they won’t process well).
I wasn't aware they were making pellets out of kittens these days, but i imagine your fish will be healthier long term if you choose foods designed for their nutrition. They may eat other foods but that doesn't mean they're going to get the nutrients they need from it.