Good subject:
Some thoughts, If your tank is outside waiting to installed but you want to paint it, mask off the top opening, tape seal it well, you can use a good spray Krylon, or even the cheaper versions. This will give you a solid look, depending on the color used, you can also go with layers of colors in shades of blue, blacks to blue etc, etc. One thing to think about eventually in a matured tank, that part will be covered hopefully with coralline. If Inside already and you can't move it again, I'd recommend using a latex on cleaned glass, despite what some say, it will adhere very well, & worse case you could use a blade to remove it. Clean glass well before use, good ol house latex, using a small roller, with two coats will cover glass well, with little fumes as compared to trying to spray from a can, also other than drips, you won't get overspray all over your living room (ok I've tried at a great expense, It was bonehead move on my part, I was anxious, this is a good example of not having patience), I didn't cover enough of the flooring, furniture etc. I was lucky the floor needed replacing anyways LOL!