I can't think of anything that is specifically aquarium safe. Mast paints today have had most human toxins to make them kid safe. As we all know human and fish toxicology are different. That being said, use any child safe or acrylic based (or even oil-based) paint to paint your decoration, then cover it with an acrylic or polyurethane clear coat (several coats with sufficient drying time between coats) and you should be safe.