Fish are capable of play

[Rob]

Chalk another one up for researchers discovering what aquarists already know: A fish's behavior is not always predicated on basic survival tactics; Fish play for the sheer pleasure of the activity. If your fish is doing something inexplicably silly, don't just assume it's a derivative of survival behavior (e.g. feeding, mating, territorial, et al.). It's possible your fish is goofing around.

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[CarmieJo]

Pretty interesting!