Pointless effort

Question

There is a saying in Japanese "kappa ni suiren" which translates as "swimming lessons for a kappa". (A "kappa" is a mythical river imp / water dwelling monster). The only similar expressions that I can think of in English are: "Teach your grandmother to suck eggs!" (which I haven't heard anyone use in a long time) and "Carry coal(s) to Newcastle". I'm looking for an "refran" in Spanish that conveys this sense. The sort of thing that can be said to someone who has just tried to "teach/explain" something that you were already well aware of; along the lines of "Tell me something that I don't know!" (but a "refran" not just a translation).

Answer 1

steve said:

Would'nt "preaching to the choir" sound closer in english?

That has some overlap but I think it's more concerned with the idea of people who don't need to be convinced of something because they already believe it rather than the idea of something that they already know.

Answer 2

What about "llevar agua al mar"'

Answer 3

Would'nt "preaching to the choir" sound closer in english'