Vinegar works well for hard water stains, a solution of 1 part bleach to 20 parts water is good for sterilization. For dried on algae fill with hot water, and let it soak for a few hours. An algae scraper, or a razor scraper will remove the somewhat softened algae.
For tanks that look like they were pulled from the bottom of the ocean, Sno-Bowl toilet bowl cleaner removes nearly anything. You have to be extremely careful, it is concentrated hydrocloric acid. Goggles, long sleeves, and ventilation are needed. Outdoors is the best sort of place for this cleaning, as you need to rinse the tank like you've never rinsed before. Do not get any on bare skin, it will do what acid is known to do.
I've used all the above methods with success, the only ones you may not get totally clean are ones etched from hard water stains. I imagine you could buff it out with an extremely fine abrasive, similar to a scratch, but it would take a lot of work.