For what its worth, I've always used high gloss latex paints. I don't think it makes a noticeable difference in appearance looking through the tank if you use flat/matte, semi or high gloss. I just like the slick surface for wiping off dust and the occasional splash back there. I feel like flat paint is more likely to saturate and get soft when wet, where the gloss is more protected/ water repellent. I have painted the backs of all my tanks, some are over 10 years old. Either spray or brush work and looks the same once its done. Brushstrokes aren't noticeable after the last coat or two. Just go thin and let it dry completely between coats no matter what paint you use. Repeat until opaque.