Fish food is not okay for cats as a regular part of their diet. However, should your cat eat fish food by accident, they likely will not be seriously ill. Fish food flakes are typically made out of fish meal, squid, earthworms, and minerals. Since cats do eat fish and sometimes fish byproducts, it is not inherently serious if your cat eats some. Keep in mind, though, that fish food of course is not formulated for cats, and may also include preservatives and dyes.