It would be best if you placed your fish leftovers in a proper container and seal. Keep in the fridge for not more than three days. It is somewhat the maximum number of days for the fish to be safe for consumption. You can stretch it on a fourth day if everything looks and smells okay, but don't push the limits. Freezing cooked fish isn't a good option because it will ruin the texture and the flavor. If you need to freeze fish for a more extended period, rather do it while it is fresh and raw.