First thing to do is fill with water, add a generous portion of bleach (enough to make the water feel slippery), and then let it sit (overnight even). This will soften and burn up the dried on bio film. A plastic pot scrubbie works well to clean everything off. If there are any areas that have mineral deposits, straight vinegar will dissolve them or soften them for easier removal. It will also neutralize any residual bleach. Once cleaned, rinse well and you are ready to go.
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