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.
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Intel(R) Core(TM)2 CPU 6700 @ 2.66GHz (2 CPUs)
2046MB RAM
SAMSUNG HD320KJ hard disc, 298.3 GB, NTFS
Windows Vista Home Premium (6.0, Build 6001 SP1)
DirectX 10