A small section of the glass is what I would be painting, and it's either paint a small area or tear apart the actual tank. I'm still debating painting the inside. I want my crazy idea to look good, so I might just go through the extra work of tearing the tank apart to paint it on the outside rather than just taking the easy route and painting it on the inside. But it's good to know about the krylon/kryon
If its a small piece or area you could just use some(thin) black acrylic and either tack it in place with super glue or wedge it in depending on your situation.