Whoa, time out!!! I wouldn’t go anywhere near a tank with a brush or roller. Spray paint is the only way to fly. Here’s how I did a 10G with outstanding results.
Clean the glass (no paint thinner, just vinegar and rinse well with water and dry)
Carefully mask the tank so that overspray doesn’t mess you up. (Newspaper and masking tape work great, and you can put smaller tanks in a plastic garbage bag and leave just the back exposed…tape carefully to seal it.)
Spray with paint made for glass (ask at your hardware store, they’ve got it) I think black looks best.
Wait for the first coat to dry and add at least one more
I’ve heard that engine block paint works great on glass, but I haven’t tried it. Glass paint, as I said, worked extremely well for me.
Let the paint completely dry before removing the masking to avoid messes.
Hope this helps!