I personally just used black vinyl, the stuff used at sign shops to make signs, its dirt cheap goes on easy and can come off again very easy if I ever change my mind. I applied it to the back outside of my tank, took under a minute and looks nice and clean.
anything black should work really. i noticed dirt showed up a lot easier on the black background. i used a matte black paper which may or may not fade over time. very simple, hung on.
if you have easy access to the rear you can stick on a vinyl/plastic sheet. any sign shop can cut an appropriate size. anything stick on you will need to clean the glass thoroughly and have no air bubbles for best appearance.