painting back wall of tank

[Canadian]

Just use a can of enamel or acrylic spray paint. Make sure the glass is nice and clean (use rubbing alcohol to remove any residue and make sure you keep the area you're going to be painting in clean so you don't get any stray lint sticking to it). And mask off the rest of the tank (including taping the top of the tank closed; you wouldn't want to get paint inside the tank and have it stick to the silicone). And do several light coats rather than one thick one or you'll get some runs that will show through from the front side of the tank.