i just make sure the tanks clean and dry, tape anything you dont want paint on and hit it with some krylon. many light coats are always better then a couple heavy coats. ive heard you can also use a paint roller but ive always used spray paint as i have so much of it and its cheap. i dont know the deal on acrylic as i always use glass tanks. i know if you get sick of it on glass you can scrape it off with a razor blade, i would imagine on acrylic this would scratch it.
again, im a fan of black backgrounds myself. i figured since its usually black i would give my QT some flavor. when the fish comes out i might scrape it off and paint it black and even do the sides since it is a qt.
no one wants to even guess what the next fish is going to be? i guess i just have everyone stumped.