Grammatically it is OK. But it's not something you would usually say. You would only use it in a limited number of contexts, usually if there are several smallish fish to choose from, and you do not eat one of them. Usually, we would say 'I don't eat fish', if fish is something you don't eat. Or 'I don't eat the fish' if the fish dish is made from a larger fish. What idea are you trying to express? Can you say what you want to say in another way?