not testing for iodine and dosing it is not being "meticulous about correct dosages". you have no idea how much is being used. you are adding plenty with your water changes already.
i would also look to the ocean water. the live rock that is aquacultured is done so miles off shore. i'm sure it would be easier to aquaculture this rock 30 feet from the beach, but they don't pick spots that close for the reasons above. the shore lines work very much like a protein skimmer in that the ocean expels it's waste there. i don't think it's unlikely that you would get bad water from there on multiple occasions.