at an angle, the bottom will show a little reflection. but its nothing that should have you thinking of other ways to color your bottom. use a latex paint, itll take a couple coats but it peels off clean and easy.
I only use Mac Tac. It's plastic and adhesive on one side. It's usually use in kitchen cabinets. Great for tanks background. Easily removed with no dirt on the glass. Can be found in all home renovation stores. I would never use anything else.