Before you begin your odor-eliminating endeavor, consider this:
Black mold -- If you see a spotty black substance on the inside walls of your tank, this could be black mold. This type of mold can be a health hazard, so clean the tank immediately and make sure you get it all.
Resorting to bleach -- Occasionally, the problem might be too intense to solve with mild cleansers, and bleach might be necessary. In this case, start diluted, with a cleanser of one part bleach to 10 parts water. Only increase the strength if you find you need to.
Protection -- When working with bleach or other harsh chemicals, be safe about it. Wear gloves and a mask, make sure the bathroom has some air flow (an open window or a fan is fine), and never mix multiple chemicals in the tank.
Quick fix -- Company coming over in an hour? Strike a match in the bathroom to temporarily mask any unpleasant smells with the less-unpleasant (and to some people, downright wonderful) smell of sulfur dioxide.
And the best approach of all to freeing your bathroom of a musty tank smell? Once you clean the tank, follow the prevention method. Getting in there a couple of times a year will help ensure you don't have to do it in a pinch.
To learn more about bathroom odors, mold, and other household cleaning topics, look over the links on the next page.