If you google iodine, all that is written is it's lethal. even at very low dosage. I did read it could be removed or absorbed by activated carbon. from what I was reading I would be nervous using it. If nothing else, fill it several times allowing the container to soak for several minutes if not hours before dumping and refilling it. Then fill it with known safe water and put a small fish in and see what happens. If it's going to react to the iodine I would suspect in the container 12-24 hours would tell if it is leaching any iodine back out. Personally, I think I would try finding another barrel. I had good luck asking a carwash, they gave them to me for free, It's a soap chemical but I did get lucky and the one's I got were bio-degrabable and after three or four rinsings and two soaking I used a small fish for the test and it made it ok over 24 hrs. Mosquito fish work great for this, They will die quick if there is a issue! Available at most any ditch, lake, pond or stream in your local neighborhood and they cost $0 But try not to get a start of them in your pond, They are hard as heck to get rid of once they are in there. 1 female can produce 100 fry and beleive me, those one hundred fry acn turn into several thousand in just one season! Good luck