zul, you inspired me to do some testing
i had a scrap of acrylic, (super thin) and sprayed it with regular black high gloss paint, the finish did come out pretty good, but it is still a little tacky and i think it would ruboff, i tried the same paint on the back of a 20 gal glass tank, it came out very nice, i think i will definatly spray the 55 gallon black, hopefully this will hide the overflow, heater, ect.
i do know one of the cheapest paint i have seen for plastic is called "fusion ", it is made by krylon, and is specifically made to bond to plastic surfaces.
thanks for idea zul, and hope this helps