His FeSO4 approach should be similar to @_Maq_ using FeCl and Maq can probably shine more light on this topic.
I've tried using both.
When you put FeSO4 in contact with (oxygenated) water, a precipitate of Fe-something (supposedly Fe(OH)3) appears instantly. It's not a powder but "something brown" (emulsion) floating in the water column. We know it from nature as a typical "iron water".
When you dilute FeCl3, nothing like that happens. Frankly, I don't quite know why. The difference is that here the iron is already trivalent (in contrast to bivalent, reduced iron in FeSO4), so it doesn't oxidize. Stock solution of FeCl3 is stable.
FeCl3 is my iron fertilizer of choice.
I've experimented with Fe-citrate, too. The nice thing about Fe-citrate is that citrate forms various complexes with iron - both bivalent and trivalent - at the same time. Add to that citrate is a
natural chelator, unlike EDTA & co. For me, in my conditions, it does not work any better than FeCl3, and the stock solution stability is somehow limited; I suspect bacteria get present and transform the chelate in their own way.
I've also made an interesting experience with
Ludwigia glandulosa in pH between 7 and 8. It suffered iron deficiency, and I've attempted to help with FeCl3, Fe-citrate, and Fe-EDTA, one after another. Nothing worked, while
Ammania crassicaulis remained healthy (even with unchelated FeCl3). So, there was no
absolute lack of iron there, but some species simply could not take up or utilize iron no matter what I've tried. (Unfortunately, I don't possess any "stronger" iron chelates.)
All that I've said is subject to the real masters of our tanks - the microbes.