I had many misconception about iron when I started my first planted tank 6 months ago, and after reading extensively and recognizing my plants were starving of iron, I finally realize why I even need dosing iron. Here are my 2 cents about iron.
Iron stands alone as the macro of micros, as plant need of iron is several to 1000 times greater than other micros. Iron ought to be elevated as the micro of macros, just below phophate.
Despite iron being the most abundant mineral in many rock and substrate, fish food and poop, it is largely bio unavailable to plants and the most commonly deficient nutrient encountered. So don't fool by commercial substrates that list the hign % of iron content which is largely unavailable to plants.
In oxygenated water, iron precipitates out quickly as Fe3, and iron in rock and substrate is bound tightly as Fe3. Natural Fe is available to plants only if it is reduced to soluble Fe2 by anaerobic reduction in the substrate or photo reduction in the water column.
This is why dosing of chelated iron is needed to solutionize Fe2 and Fe3 so plants can absorb it, but you need to dose the sweet spot to prevent algae outbreak or poisoning plants, fish or inverts.
The choice of chelate and frequency of dosing is also critical because the stabilty of chelate may break down instantly or in short time depending on the type of chelate.
I dose gluco Fe and DTPA Fe because it is more stable in my neutral to alkaline water. The stability is only short time, so I have to dose every other day. For acidic pH, edta Fe Is fine
