I don;'t think iron is ever worth measuring in a marine environment because it never needs to be high enough to detect.
I discuss iron dosing here:
First Iron Article: Macroalgae and Dosing Recommendations
http://www.advancedaquarist.com/issues/aug2002/chem.htm
Second Iron Article: Iron: A Look at Organisms Other than Macroalgae
http://www.advancedaquarist.com/issues/oct2002/chem.htm
from the first one:
Iron in Reef Tanks: How Much and What Form?
Deciding how much iron to add is fairly easy because, in my experience, it doesn't seem to matter too much. Presumably, once you add enough to eliminate iron as a limiting nutrient, extra iron does not apparently cause harm (at least that I've detected in my tanks or heard of from others). I selected a dose of about 0.1 to 0.3 mL of a solution containing 5 g of iron (as 25 g of ferrous sulfate heptahydrate) in 250 mL of water containing 50.7 g of sodium citrate dihydrate. This liquid is dosed 2-3 times per week to my system with a total water volume of about 250 gallons. This iron(II) citrate has turned brown and cloudy since I first made up the bottle years ago, suggesting that it is oxidizing to iron(III) and some is precipitating from solution, but I still use it. Over the past 4 years, I've dosed nearly all of the 5 grams of actual iron to my tank.
Now that may sound like a huge amount, and it is. It's enough to bring 800 million gallons of completely depleted seawater up to the 0.000006 ppm level that I mentioned earlier for natural sea surface water. Still, I've not noticed any problem, do not know the steady state concentration, do not know how high of a solution concentration is actually optimal for my tank, do not know how much is biologically available by the mechanisms mentioned below, do not know how fast it is removed by skimming and other mechanisms, and do not know what would happen if I cut it back by a factor of 1,000.
All that I know is that microalgae has never been a problem since starting the iron, and I've not noticed anything negative that I could attribute to the iron (nor have I heard of any from others doing similar dosing). Still, I don't keep all organisms available to the hobby, and if you do seem to get a negative reaction from something, I'd advise backing off on the dose or stopping completely.
Since many hobbyists do not have access to the chemicals required to make iron(II) citrate, I'd advise buying a commercial iron supplement. There are a number available that seem appropriate and are not very expensive. Some commercial supplements combine manganese with iron (such as Kent's product), presumably because the scientific literature has demonstrated that phytoplankton also scavenge manganese from the water column. I've not experimented with manganese, but it is probably fine to use if you cannot find a pure iron supplement.
I'd also advise using only iron supplements that have the iron chelated to an organic molecule. The iron sold for freshwater applications is sometimes not chelated because free iron is more soluble in the lower pH of freshwater tanks. I'd avoid those products for marine applications. It will likely still work (as many of the studies in the scientific literature use free iron in seawater), but probably not as well because it may precipitate before it has fully fortified the system with iron.
In many cases of iron intended for the marine hobby, the product may not tell you what the iron is chelated with, in order to protect proprietary formulations. I don't actually know if it matters too much. Very strong chelation by certain molecules will actually inhibit bioavailability by not permitting release of the iron without completely taking apart the chelating molecule, but I expect that manufacturers have avoided those molecules. EDTA and citrate, and some others, actually degrade photochemically, releasing small amounts of free iron continually. It is believed to be the free iron that is actually taken up by many organisms, and likely iron(II), though some organisms may be able to convert iron(III) to iron(II) before uptake (the detailed absorption mechanisms are generally not known). There is a more detailed discussion of this degradation and uptake in "Captive Seawater Fishes" by Stephen Spotte (1992).
So good luck with iron dosing, and happy reefing!
