The big three in reefing will always be, IMHO, calcium, alkalinity, and magnesium. If you're telling a newbie to buy additional test kits, please make sure that they already have these in their quiver.
With that said, iodine test kits ARE available. They just aren't very trustworthy or simple to use. Putting too much faith in the results and overdosing iodine can be disasterous.
-------RANDY HOLMES-FARLEY-------------
http://www.advancedaquarist.com/issues/mar2003/chem.htm
Measurement of Iodine: Test Kits
There are many ways to measure iodine in seawater. Even confining the list to those that most aquarists are likely to encounter, there are still at least three fundamentally different ways, and those three all detect different subsets of the total iodine species in water. Unfortunately, those differences make comparisons of the different methods almost impossible, and also complicate the interpretation of the data from any single one of them. Of the commercially available kits, the Seachem and Salifert iodine kits are the most popular in the US. Unfortunately, the use and interpretation of these kits is tedious and complicated. I’d like to hold off on detailed comments about how well they work until I’ve had additional time to study them, but so far my experience has been rather less successful that with simpler kits (alkalinity, calcium, etc.).
The Seachem iodine kit detects only iodide (I-) and molecular iodine (I2). It is unlikely that there will be much I2 in an aquarium as it quickly breaks down into other products59 [to be described in a later article that will include a discussion on using products that contain I2, such as Lugols solution]. A potential drawback to this kit is that it doesn’t detect iodate. So if your tank water matches normal seawater in quantity and speciation of iodine, then it will look artificially low (say, about 0.01 or 0.02 ppm). I would also not advise using this kit if you are adding an iodine supplement that contains considerable amounts of iodate, molecular iodine (that may break down into products that include iodate and iodide)59, or organic iodine forms. There is also the concern that supplemental iodide, and those forms of iodine coming in with marine foods, may end up partially as iodate. Consequently, this kit may substantially underestimate the total amount of iodine present. I fell into that trap years ago in dosing iodate to my tank, and a significant concentration built up before I specifically tested for it.
The Salifert iodine test kit detects iodide, iodate, and molecular iodine. Salifert is about to come out with a new iodine kit (using a yellow color instead of a pink color). Assuming that it functions properly, it would be a good choice for anyone dosing iodide, iodate, or molecular iodine, though it is a long test involving quite a few steps. It will not detect many organic iodine forms, and people dosing such compounds should beware of overdosing.
-------RANDY HOLMES-FARLEY-------------
http://reefkeeping.com/issues/2007-04/rhf/index.php#17
Supplementing Iodine:
Many aquarists dose iodine, and claim that certain organisms need it to thrive. Often mentioned are shrimp, Xenia species of soft corals, mushroom corals, and more. However, no evidence for an iodine requirement by these organisms appears anywhere in the scientific literature. They also thrive quite well in many coral reef aquaria where iodine is not dosed. Of Reef Central’s Tanks of the Month for the past couple of years, the majority do not supplement with any form of iodine (or at least do not mention doing so), although some certainly do dose it.
I do not presently dose iodine to my aquarium, and I do not recommend that others do so, either. Iodine dosing is much more complicated than dosing other ions due to its substantial number of different naturally existing forms, the number of different forms that aquarists actually dose, the fact that all of these forms can interconvert in reef aquaria, and the fact that the available test kits detect only a subset of the total forms present. This complexity, coupled with the fact that no commonly kept reef aquarium species are known to require significant iodine, suggests that dosing is unnecessary and problematic. On the other hand, it is nevertheless possible that some organisms that we keep do actually benefit from iodine, and that in some aquaria there is not enough in the foods that we add so that supplements may possibly be beneficial in those aquaria.
I dosed iodine for several years when I first set up my aquarium. I dosed substantial amounts of iodide to try to maintain 0.02 to 0.04 ppm of iodide (which is a natural level). Iodide is rapidly depleted as algae and perhaps other organisms take it up and convert it into organic forms. After a few years of dosing iodide, I became frustrated with the complexities of testing for it, so at that point I stopped dosing any supplemental iodine. That was about seven years ago. I detected no changes in any organisms, and never dosed any again. If you are dosing iodine now, I suggest stopping for a month or two, and seeing if you can objectively detect any difference in any organism.
For these reasons, I especially advise aquarists NOT to try to maintain a specific iodine concentration using supplementation and test kits. For those who do supplement iodine, I suggest iodide as a more suitable form than certain other additives, such as Lugol’s iodine, which is unnatural and potentially more toxic. Iodide is also more readily used than iodate by some organisms, and iodide is detected by both currently available iodine test kits (Seachem and Salifert).
-----------------------------------
Here's a 2008 thread in which bertoni and Billybeau1 both expressed clear support for Randy's position on the subject. These are three of the most prominent reef chemistry gurus on the net.
http://www.reefcentral.com/forums/showthread.php?s=&threadid=1303831
I had some links but they never amounted to much. Plus as I mentioned above, most of us that posted in those threads, own professional built turf scrubbers, {although mine is modified as it arrived smashed to bits}, {it was purchased 2nd. hand}. I dont believe any of us are running them at the current time. 
