Outside the tank, regular enamels are ok...if it's going to touch the water the Krylon Fusion like Euge said, is best.
Tip (1)...Get a piece of glass to practice on, you'll be able to see how much to put on for the look you want.
Tip (2)...I paint with several colors (black, brown, primer red & gray, gold) and paint in stripes (vertical or horizonal) or spots to give texture (like the sides of a river bank or bottom, very abstract or blurry).
Point (1)...If you paint glass it makes it very reflective (mirror-like), I've had some fish that are very paranoid and don't realize there looking at themselves.
Tip (3)...Unless you want the above effect, dust the spray paint on in several coats, letting each one dry before the next one. NOTE: This makes for a delicate surface (scratches easily).
Hope this helps,
Dr Joe
.
