I just grab a can of Krylon from the local hardware store. Any kind will work just fine though...
Just clean the surface with alcohol let air dry, tap off the rest of the tank and MAKE sure you cover the top. You'd be surprised how much over spray comes from a little can TRUST me. And if you can, I recommend doing it outside or in a well ventilated area like a garage.
"When creating an aqua scape, strive for the beauty of imperfection, as its art and there is no wrong or right, it's left open for interpretation" James Findley