I test for alkalinity, calcium and magnesium.
In 12 1/2 yrs, I've never fed corals other than the yellow cup coral that needs to have each individual cup feed.
I feed my tanks heavy for the fish, and the inverts/corals etc. get what they need from that.
Iodine tests are not that reliable and there is no documented proof that iodine must be added to your tank in addition to that added via water changes or via the food you put in the tank.
If you DO add iodine, it is possible to crash the tank with just a little too much.