paint

I’m cleaning up a tank for a friend of mine and am going to paint the back. The only problem is is that there are hard water stains on the back. If I paint the back will the hard water stains mess up the paint?

I have built a small structure to sit on top of my aquarium and hold a large plant tray. Water will cycle into the tray and back to the tank. However should a leak ever occur which is likely, I do not want the wood structure to rot, or leech chemicals into the tank. I would probably need at...