Not sure what would cause a white deposit. You might try tumbling some heavy nuts/bolts in the tank to knock the stuff loose. I've heard of folks using gravel and short lengths of chain as well. If that doesn't get it you'll probably have to do an acid clean and re-line it or replace it. Do a site search for tank cleaning. You'll find a LOT of threads on the subject.

Save a life, adopt a senior shelter pet.
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Name your dog Naked so you can walk Naked in the park.