I can see why you would want to paint your inside of the tank if the back was already painted. I had this dilema bright blue back. I thought about adding in some plexi pieces to the back. I could paint the plexi on the back and attach it to the back wall. It's an idea. That way you wont scrape off the paint.
I ended up sanding off the paint on the back of my acrylic 100, and painted it black. And like an idiot I let the sun hit it one morning while it was in the garage drying and it heated up and cracked. (Black paint)
If the tank is plexi, you can sand it and paint it, and the scratches are not noticable. try it on something like a CD case. Sand it, and paint the sanded side. Look at the other side. Cant tell.
Anyhow, what is your goal?