I painted my 55g blue, don't like it much. Partly my fault because the airbrush messed up and I finished with a brush. So the paint turned out uneven, some of the lightly coated areas look paler than the heavier ones. For the next tank I tried a piece of framer's matting material (more textured than posterboard). So far so good, no dripping problems.
Window tint film sounds like a good option if affordable, it comes in a lot of shades.