Well, if you add enough vitamin C, the iodine or clorine will stop reacting and you'll have left overs. As for the plastic taste... why do you think I use Nalgenes? The only time I've ever had a taste in one was when I kinda left one about a quarter full of ice tea in my car for a week... that needed some time to air out.

As a side note, when your ice tea is furry, it is best to let it run free.