I painted it black with latex paint. Took about 5 coats before it was covered to my satisfaction. I really like a black background on an aquarium. It really makes your plants and decorations pop IMO.
Your tank looks awesome and your pics are fantastic.
I got a black vinyl backgrond for my granddaughter's 20G but now I can't think of how to put it on there without it looking tacky. At least I have black sand in it. LOL