"To paint or not? " I would say Not. Why? Glass, by nature of its smooth surface, is not a desireable surface for paint to adhere to. If you have cords, cables, wires, hoses with clamps, hang on devices etc, that will dangle and possibly scratch the paint, it now becomes an eyesore. I have always used a black of blue piece of aquarium backing from any LFS. I don't like using glue as it is nearly impossible to get it perfect with an airless finish. Instead, I will measure and cut the backing both vertically and horizontally, leaving 3/8" or so excess on the top and bottom cut. It your tank has a black plastic edge, as most tanks do, I'd slide the top upwards and underneath this edge starting at the top then sticking the bottom edge inside the black trim. I have done this with every tank I have ever own and it looks greats. But now you have to seal it to prevent any splashed water from dripping between the glass and the backing. I use basis 1" clear tape to seal all four edges. It is a flawless and seamless finish. Make sure you don't crinkle your backing as it will show through when viewed from the front of the tank. Good luck my friend.

Just my 2 cents.

Mucho Reef