Fish is not inherently bad for cats, but a diet excessively high in fish can lead to nutritional imbalances. Fish, especially if fed in large quantities, can result in thiamine (Vitamin B1) deficiency in cats, causing neurological issues. Additionally, certain types of fish may contain mercury or other contaminants that can be harmful. It's essential to provide a balanced and varied diet for cats to meet their nutritional needs. If including fish, ensure it's cooked, boneless, and offered in moderation as part of a well-rounded feline diet.