mask off everything that you dont want to get paint on (masking tape and newspaper works well) and grab a can and go at it. obviously you want to paint the outside. Its better to use a lot of really really light coats, because any runs will show up. I just used enamel on mine and it worked great. Goodluck and post the out come!
Yea, youre probably not going to want to spray the tank inside anyway, i just assumed this was a new tank.....you could try to brush it, but you would probably see all of the brush strokes. Maybe you could just use a black posterboard.....it would look almost the same as a painted tank.
I used limo tint on the back of my aquariums. It's dark enough so you can't really see the back, it's less messy, it's easier, and I got the tint at Wal-Mart so it only cost like $11.