I have two great oscar "caves" made from 10" PVC pipe from my beet farmer neighbors. Only problem is they're ugly as sin. I'd like to paint them, but scared of paint toxins leaking into water. Any paint available for that, or do I just stick with ugly but wonderful caves?

Marine grade paints are the best choice. They are usualy available from boat supply shops. No doubt you can find them online as well. They come in two parts that you mix before you apply it, then it sets up over a few days. Totally fish safe and water proof.

personally i would never out anything 'paint like' in my tank cos id also be scared of the toxins but if u can get some marine based ones as mentioned it could be a plan. got nay pics? before and after would be good!

i'd be afraid the paint would flake or chip off and be ingested by the fish

not quite sure what you mean by cave, but here's an idea - I've seen epoxy type adhesive that is safe for aquariums (drs foster and smith online); perhaps you could somehow cover the PVC with substrate/ gravel??- that way it would blend in with the rest of your substrate? of course you'd have do do this outside the aquarium and let it dry/cure for a while

paint it with aquarium silicone, then pack that silicone with some well dried substrate.

Be sure to use 100% silicone. Also if you get marine grade paint be very careful the fumes are not something to mess around with. The silicone will stink to high heaven but its not harmful.