I've got a glass tank. I bought Olympic semigloss exterior something-or-other in a color that I liked at Lowe's. I used a small roller with a very short nap to apply. I applied 3 coats. Don't worry if it looks sort of ugly, because when the tank is lit, the light reflects from the inside of the tank, and it doesn't matter that the paint looks ugly on the outside.
I disagree with using any gloss paint; flat adheres to the glass much better. From inside the tank, it looks glossy because of the glass.
Rustoleoum flat black enamel works great. Just make sure you clean the glass really well first (use vinegar then water), as even dust will show through.
Spraying is the best way to apply (mask off other sides and inside), but rolling is possible in 2-3 light coats.
The most important thing, though (to me) is to use a water cleanup paint. Look on the label of the can for the cleanup instructions, and use a water cleanup, not paint thinner cleanup. I can't stand the smell of paint thinner...