I second the white vinegar. Fill it up with water and add cup* or two of of white distilled vinegar. It is available in the grocery store for $3. Then Scrub it off or let it sit for an hour and it will fall right off.
*(Only if there are no fish in it.)(unless you want dead partially-pickled fish.)
(Donning Lab coat and cute safety goggles) "The white stains are minerals, mostly calcium, that have accumulated as water has evaporated out of the tank. When these minerals are in the water they serve as a buffer to help control pH. Any liquid that lowers the pH sufficiently will cause them to dissolve quickly. Regular water will also dissolve them, but through a different mechanism and not nearly as quickly."