As far as aquarium safe paint, caulk, rocks, (fill in the blank), I'm a bit reckless. Right now all the PVC in my tank is painted with cheap-o generic black spraypaint. All the silicone in my aquariums is just GE standard 100% silicone sealant. If I were terribly worried about the paint, then I would honestly just take a PVC pipe, paint it, clear coat it, stick it in a 5 gallon bucket. Put a couple shrimp in there, along with maybe a kuhli loach or something else which can be sensitive to water parameters. Make some sort of cheap, diy filter for the thing out of what you have laying around, and some used material. Wait and see what happens.
The only spray paint I wouldn't be willing to put in my tank without trying it out first would be anything that is paint, plus something else. Like the spraypaint which is supposed to look "antiqued" when it dries. There has to be some weird chemical reaction to make paint do that, and I'm not sure what chemicals are in there. Same goes for chalkboard paint, whiteboard paint, magnetic paint, etc.
My reasoning behind this is fairly simple. There isn't THAT much paint on anything you're putting in the aquarium. Of the little bit of paint that's in there, it can't be leaching too much stuff, otherwise the paint would disappear.
I know this is more of a ******* approach to whether or not something will work as planned, rather than a scientific appraoch, but... if it works, it works. I've never had any problems with anything I put in any of my tanks with the exception of some rocks which made hair algae go absolutely nuts. I removed the rocks, and the hair algae disappeared.
My theory is that all creatures are designed, and built, to LIVE. Critters get sick, and do sometimes die, but most animals put up a fight before buying the farm. During the fight, move the thing that is making them sick.
In closing, I would just buy whatever clearcoat was either cheapest, or looked the best (maybe a high gloss, or something?) and give it a shot. Also, no promises that the copper paint won't corrode. Clear coats aren't waterproof, as many peolpe think. If your concerned about the clearcoat, or about the copper paint, you could go with some clear two part epoxy. Totally aquarium safe, and totally waterproof.
Sorry 'bout the novel. I like to write! (maybe I should just add that to my signature... I say it an awful lot)