Regular old house paint works marvelous... It goes on easy and is easy to remove.
Spray paint works, but overspray becomes a major concern durign application and removing the spraypaint is a lot of work, very messy work.
Krylon Fushion is recommended for submerged purposes. But for the outside of the glass it's just overpriced spraypaint. Also it's worth pointing out that the manufacturer of Krylon Fushsion does NOT state anything abotu it being fish safe and DOES state that their product has in no way been tested on animals. But it has been proven by us hobbyists to be safe. But that makes me wonder how many other products are just as safe...