They're 'mosquito' larvae (they might not be actual mosquitoes, but some other sort of small flying insect; all the larvae look pretty much identical to us non entymologists!).
While I'm with the others on saying they are harmless in a tank... Either get rid of them quickly, or put them in a tank as fish food quickly.
Looks like mosquito larvae to me, and you do NOT want those things developing, flying around, and biting you! So find some way to make them disappear fast, before they become adults!