so a friend of mine is a painter and askewd if he can paint a background for me. My plan was to use a plastic sheet and have him paint it and then drop it into the tank so my question is, was paint is safe for the fish and wont contaminate the water with unwanted chemicals?
I don't think there are any aquarium safe paints, and if there was, I wouldn't trust them. Scrape the paint off the back of your tank, then have your friend paint a tarp or sheet, then hang that on the outside of the back of your tank.