So it's corrosion then... ouch.
If you've got an inspection port in the tank, you'll probably want to open her up and check out the inside thoroughly. If you don't have an inspection port you may be able to get a mirror in through the fill hole or some other opening. It'll be a pain in the kiester, and may require draining the tank. Unless you can be absolutely sure you've removed all of the corroded metal, this may become a recurring project.
Hope all works out well.