"Teaching fish to swim"

Solution 1:

Teaching grandma to suck eggs, or teaching your grandmother to suck eggs is the unequivocal idiom which means giving advice to someone who is already an expert in the subject or field. There's also the implication that the "teacher" in question is less-experienced than their pupil.

Teaching fish to swim, although easily understood and to my ears, more contemporary, (who sucks raw eggs nowadays?) I don't think it has entered the vernacular, not yet anyway. According to Wikipedia

Teach fish how to swim is an idiomatic expression derived from the Latin proverb piscem natare docem. The phrase focuses attention on the self-sufficient perception of those who know how to do every thing better than the experts. [...] A corollary idiomatic phrase is part of common usage in Chinese (班门弄斧)

Solution 2:

Apparently my stack exchange reputation doesn't carry over between sites, so I can't comment on previous answers. I just wanted to add that "preaching to the choir" is more about not needing to convince someone of something, because the proverbial choir is already on "team jesus".

Also, "teaching [one's grandmother] to suck eggs" strikes me as very region-dependent. If someone said this to me, having grown up in southern california, I'd have no idea what they meant. I mean, I would now, because I read the other answers. But I wouldn't have 10 minutes ago. Could be misconstrued as something vulgar, or maybe a very awkward non-sequitur.

"Teaching fish to swim" is probably closer to what OP wants. It might not be part of everyone's common parlance, but it's at once clear what the speaker means, and doesn't feel at all awkward.