The iron is, I believe, always below testable levels, so I do not test for it, nor do I consider the level very important.
First Iron Article: Macroalgae and Dosing Recommendations
http://www.advancedaquarist.com/issues/aug2002/chem.htm
from it:
"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. "