Choose a nice shade of blue or whatever you like and spray it on the outside. Cover and mask the whole tank top so no paint gets into the tank. The paint shouldn't really matter since it's on the outside and does not come into contact with the water.
Sign Painters' 1shot is great. Comes in black or different shades of blue and covers much better than regular paint. Call (219) 949-1684 for the dealer in your area.
I have done my tank with Red Devil oil paint brush on I have also tried same brand acrylic.
Place the over tank light on the tank to make sure you covered the back completely. Use a brush. stay away from the sponge.
_________________ Colt Roberts
rustoleum (black or blue) with a small roller has been my time-trusted trick. That way you don't have to mask anything, you get an even coat and won't see brush marks. Several coats is a good idea.