Thanks for the answers.
Of course I mean pure white vinegar, no balsamico or other fancy food-aromatizing substances.
My only fear was there problably are bacteria or other microorganisms that use acetic acid for food. Still not clear if these live in aquariums too.
Apparently everyone knows vinegar is useless but nobody can tell why… The fact that other mixtures with acetic acid are used as food is of course not an argument.
Of course I won’t try vinegar in inhabited tanks, so, as we say here in the Netherlands: baat het niet dan schaadt het niet, and of course, niet geschoten is altijd mis.