Teach fish how to swim is an idiomatic expression derived from the Latin proverb piscem natare doces.
The phrase describes the self-sufficiency of those who know better how to do everything than the experts.
It corresponds to the expression, "teaching grandmother to suck eggs".
[1] Erasmus attributed the origins of the phrase in his Adagia to Diogenianus.
[2] A corollary idiomatic phrase is part of common usage in Chinese "班門弄斧"[3]