There are very few situations in the home aquarium where adding suppliments is really needed. Doing regular water changes will replenish any depleted trace and major elements and add the proper amount of buffering for most tanks. If you have a heavy LPS
coral load then the addition of CA
may be needed, depending on consumption. For most tanks do not waste your hard-erned money on suppliments...espcially mag. As mentioned, NEVER dose anything into the tank that you do not test for. Iodine is espceially dangerous so I would steer clear of it.