With a 4 inch gap between the full live tank and the wall, I painted the back of the tank with latex flat black enamel. Over 2 days, I painted 1/2, then the other half, then added another coat. The two coats covered well, and I just moved equipment out of the way as I needed to. I used a disposable foam brush. I was very pleased. If you are really good, you can do this without also painting the wall and the floor. I am only pretty good.
Painting in this way had no apparent effect on the life in the tank.
Be careful and it should work.