Smart enough to know who's the top fish in a school, just by watching.
Scientists found fish capable of some fancy reasoning. Cichlids could tell who would win a fight among fish in a nearby tank after watching some pairs fight. They used "transitive inference," predicting the outcome of fish A fighting fish E without ever seeing A fight E.
Hmmm...sounds like a useful skill, maybe I can get some cichlids to help me bet on football games.
This ability is equivalent to the reasoning skill of a 4 or 5 year old human. Does this mean the fish are as smart as a 4 or 5 year old? Fish are really smart at being fish, and they need to know who would win a fight. But don't expect a fish to read a Dr. Seuss book.