See if you can get your colour in a cellulose touch up can,those with a small paint brush attached to the lid or buy a small paint brush to apply the paint.Cellulose is the paint which was used before 2pac came out,its a softer paint and does'nt chip.Apply the paint in layers over a few days to fill the scratch,whan it's completely dry use T-Cut on the area to level the paint out(you might have to T-Cut the whole tank?) then give it a good polish.Used to cure problems like that on old classic cars I used to own...

Regards Sumpnut