James,
Small amount of iron in your tank should be ok or could be good. Just think about the case that the goverment shunk many old battle old ships in the ocean for man-made reef and living things start to make there home in no time. (they did removed all copper of from the ship..) but the case of old tires that they shunk in Ft lauderdale was a major mess and now they have to remove all most a million of them