If a fish lives to 20 it becomes an adult. If you haven't bred that variety before it will then count as a new breed discovered even if it promptly dies, as happened to you. The fact that it was a magic fish makes no difference. It still counts as a magic fish discovered, although, since it is no longer alive, it will not confer its magical properties on the other fish.
_________________________
To err is human; to arr is pirate.