well, i've heard of some people who paint with acrylic that they didn't seal it, but for a moist humid environment like a viv, I'd suggest sealing. As far as finding a flat sealer that i would feel comfortable using, i haven't found one yet. There are two avenues you can go with sealing. Shields All is a good sealer for vivs and will hold up to moisture just apply about 4 coats of it giving ample dry time between coats. In the final layer of sealant some people sprinkle colored sand on it to break up the shiny appearance. If your background is going to be getting alot of water on it, you may consider final sealing with silicone and again you can sprinkle sand on it to break up the shiny appearance, but when you don't sprinkle sand on it the background will look wet due to the silicone look, which if you are misting your viv it will look just like it does when water is on it from misting.