Hey there.
Use a brass brush and Mother's chrome polish to gently remove the rust. What color is your tank? If it's black, you're in luck. Take a permanent black marker and color in the spot and wax it, or if your steady handed, use a clear nail polish with some of the fibers cut off of the brush for a fine coat.

Brandy

 

After you've gotten most of the rust off, but before you paint it, you need to kill the rust that remains. Rust inhibitor comes in a few different forms. My favorite is in an aerosol can that I get from NAPA. You'll want to mask everything except the exposed metal you're treating, because it will stain the paint.