if you go for 1 month without dosing iron (you haven't mentioned whether you do water changes) and there are no signs of iron deficiency in the plant, could it mean that your substrate is rich in iron, or your water source contains some iron?
No. I rather believe it's partly because I have no filter.
You know, normally - without filtration - most of iron precipitates in the substrate. But such an iron
is not lost. Living bacteria as well as dead organic matter make iron perpetually transforming, and plants can get it through their roots. Also, humic substances and various other organic compounds function as natural chelating agents of iron & other transition metals.
The only iron which gets truly lost is that which gets trapped in the filter. There, iron oxides adsorb phosphates and other metals. As long as the filter remains well-oxygenated, these compounds are unlikely to dissolve. On the contrary, they accumulate. Haven't you ever noticed that brown powder and brown colored sponges in your filter?
I'm using solely RO+DI water which I mineralize. The content of iron and other metals is below detection. Yet I admit that I don't inject CO2 and dose all fertilizers quite modestly, so my plants grow rather slowly. Therefore, the need of micronutrients is relatively limited, too.
Still, I believe that most people hugely overdose micronutrients. I happened to study a bit an official analysis of tap water in Czech Republic, and I've learned that in most cases no micronutrient fertilizers are needed, with the exception of iron - due to its peculiar behavior.