IMO, no kit or checker for iron is really needed or useful for routine husbandry as none can detect as low as NSW levels and there is no reason to keep it higher. Even ICP cannot detect NSW surface levels of iron. ICP can barely detect the iron when I first dose it, and then later it cannot even if it is way above NSW levels (which can be only 0.000006 ppm.
This is from one of my articles on my ICP results:
Recently, Triton GmbH in Germany has initiated a method of testing many ions at once for a single price. In the US, that service is coordinated by Unique Corals. For $49 they will send you a couple of tubes to collect a tank water sample, and a prepaid mailer to send the sample back to them...
www.reefedition.com
Iron (Fe). The natural iron level varies a lot with depth, but surface seawater may have only 0.006 µg/L. The Triton LOD = 0.3 µg/L. I dose iron, and when I dose it I boost iron to roughly 1-2 µg/L, which would be detectable. This sample was taken more than a week after the last iron dosing, and none was detected as it gets depleted in the meanwhile. I’ve not yet seen a Triton test result for a real aquarium sample that had detectable iron, but that doesn’t mean these tanks are necessarily deficient. Iron is also a case where the form is critical, and ICP cannot distinguish form. Binding to organic matter, for example, can alter the bioavailability of iron.
