Painting the back wall

[Jaws]

I was just wondering what the preferred method for painting the back wall of your tank is. I painted my last one using a 4" brush and black paint and I found the paint would dry and then could easily be chipped or wiped off. Even when I did the second coat, some of the dried paint would slide off the glass. It looked really streaky too from the inside of the tank instead of a nice solid color. Any suggestions on what paint to use and what brush would be great. Thanks for your help.