The Iodine checker is for use in pools and spas where iodine is used as a disinfectant. The range it measures is a thousand times higher than iodine levels in a reef tank. Any reef water sample used in this checker would read 0.
I've tried the Iron checker, but it always gave me readings of 0. However, ICP-OES testing also generally gives me a measurement of 0, even though I supplement (lightly). I presume that supplements are rapidly being consumed and/or precipitated onto surfaces, so there's rarely any iron actually in the water. Point being that I haven't found the iron checker to be particularly useful in a reef tank so I don't use it routinely.