its always futile to test iron in the water. The amounts we need for healthy plants are so low that any inexpensive test kit isn't accurate enough. They often don't register until 0.5ppm, well above what we need (and overdosing iron seems to bring on long thread algae for most).
Eco complete isn't that iron rich...it just holds and distributes nutrients very readily. flourite (and laterite, an additive) is iron rich and slowly dissolves over the decades, releasing iron to the roots.
If your tank is very high light,
CO2 injected, you'll likely need to add a bit of iron to the water column as well. I use Flourish Iron a couple times per week, about 1 capful, for my 75 gallon. I also have some iron in my trace, Plantex CSM+B, and that's dosed on different days than iron.
Iron oxidizes pretty quickly in the aquarium, so its best to add a little 2-3 times a week, and unless you're seeing iron deficiency symptoms in your plants (
http://www.csd.net/~cgadd/aqua/art_plant_nutrient.htm) you can assume your iron is fine. Its only a trace, not a macro, so relatively easy to maintain.
