Vinegar. Just spray and let soak with plain old vinegar. Repeat if necessary. Rinse like crazy before putting anything in there, obviously, for the smell. But at least it is all natural.

I believe I live in the state with the hardest water in the US - we use bottled water for everything because it stains the terrarium glass so badly. But from my tea pot to my rice cooker, they all get a vinegar treatment every few months. Easy and cheap and works like a charm. (And just as good as those crazy chemical solutions. CLR? Is that it?)