Teach fish how to swim is an idiomatic expression derived from the Latin proverb piscem natare doces. The phrase focuses attention on the self-sufficient perception of those who know how to do every thing better than the experts. "It corresponds with the expression, teach your grandmother to suck eggs". Those who would attempt to do so are thought to exhibit a combination of hubris and arrogance in trying to engage in a needless exercise for which they are ill-equipped.
A corollary idiomatic phrase is part of common usage in Chinese (班门弄斧)
Teach fish how to swim Wikipedia
Erasmus attributed the origins of the phrase in his Adagia to Diogenianus.