Nitrate, Nitrite and Ammonia should be at 0. I have a test kit for Alkalinity and Calcium. If you have corals, your cal. should be over 400. Yes, you can test for Iodine but it's not something I've ever tested for. There are also test kits out there for Strothium and stuff but I don't consider myself an expert by any means.
I don't ad any supplements to my tank. If I have a problem I do a 20% water change. Dumping some type of supplement in your tank usually just throws off something else. Rather than puting a band-aid on something, figure out why something is too high or too low and fix the problem. I had a PH problem in the beginning. I adjusted the lenght of time I leave my lights on and added some powerheads....no problems since.