An epoxy based paint is the only thing I'd trust in a tank although I have doubts about it sticking to PVC
. If I tried it, I'd probably rough up the surface of the pipe with some sandpaper. Another option, although it may not appeal to you, is to start some star poylps, shrooms
, or zoanthids on the pipe and let them cover it. Coraline just loves PVC
too. Unless you're keeping the back glass scraped, it will be a solid sheet of coraline in time and the coraline covered pipes will blend right in.