Vitamin C would be great for colonic cleansing. The only problem is the benefit you gain from vitamin C would require a fair amount to be put into the water making it less cost effective in comparison to ozonated water or garlic.

Though i would warn about using pure l-ascorbic acid as it may burn due its high acidity. Try using an alkalised form first. Also I would suggest using warm water, not hot otherwise it just vapourizes the vitamin C.