I am inclined to agree with you but getting just iron, I have a rusty nail, or iron tablets, (for humans) but somehow I don't think they will work. I have used the iron tablets in tomato plants on my aquaponic tray. Worked there.
There is iron IN the supplement I bought yesterday, which was Flourish. I saw new red cyano this morning, wiped it off the glass and filled the CO2 generator. I did a water change last night and potted my crypts. Put plant food tablets in their pot. The crypts looked iron deficient, pothos died, I can't kill a pothos in Fort Worth water but I killed one in that aquarium, and I've putting in tanks for years. The mossy stuff died back heavily too, only a couple strands left, but the frogbit is going wild, which makes me wonder who's eating all the plant food