I went to HD and told the paint kid I wanted to paint the back of my fish tank and asked what type of paint would work best. He takes me over to his buddy and says, "Hey, this guy wants to paint the inside of his fish tank." And they both had a nice laugh. Some people you just can't reach .
I used a long-handled roller to paint black acrylic paint onto the back after my tank (glass by the way) was all set up and in place. Worked well, don't mind those black smudges on the wall ;-).