AD makes a good point. It might be worth the effort to scrape some rust off of something and see if it's drawn to a magnet. Even if it is magnetic I believe the tanks were chambered so dropping a magnet-on-a-string in your tank probably wouldn't accomplish much. It's probably time to bite the bullit and pull the tank. It's not a fun job but once you're done, you're done.

Save a life, adopt a senior shelter pet.
The three main causes of blindness: Cataracts, Politics, Religion.
Name your dog Naked so you can walk Naked in the park.