I've never added iodine. I've read several articles that discourage it, mostly by Eric Borneman and Ron Shimek. I'd be very careful, and probably a little skeptical of test results. Hobbiest test kits are notoriously inaccurate by lab standards. I'd also find out what NSW levels of iodine are and stay comfortably below that. It's possible, even likely, that your animals are depleting the iodine not because they need it to survive, but because there's some evolutionary benefit to the eco-system that it be removed from the water.