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You shouldn't scrub plastic - it will scratch it, which leaves small spaces for micro-organisms to grow, which are subsequently even harder to clean away. Instead, get a cleaner such as PBW or Oxiclean (I personally prefer PBW since it can be reused - Oxiclean loses potency once the oxygen has been released.) You can fill the pail with the made up PBW ...
My initial guess was the Campden tablet's SO2 was the culprit and hunting around for similar stories I found this on a home brew forum: http://www.homebrewtalk.com/showthread.php?t=267418 Observations in this thread would seem to fit with what you have experienced.
Iodine based sanitizers may also stain your plastic equipment. My recommendation: Leave the bucket outside in the sun for a few hours, then smell the inside. if the inside has any aromas then you need to take the bucket back or get a new one. Cheers
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