I dont think iodine does much in saltwater. From what I understand its very unstable and can be easily overdosed. Regular Water changes should give you what you need.
Salifert makes a good Iodine test kit.
Dosing Iodine:
Pros: Helps algae fend off herbivores.
Cons: Overdosing kills Crustaceans first, when levels are higher it will kill worms & snails, followed by bacteria.