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.