I think you were dosing more than could be taken up by the plants. So,adjust slightly down and when the plant mass increases- increase the iron to keep pace.
I've grown plants and worked in horticulture and have never seen iron produce such incredible results as in aquarium plants. Also,my theory is that the chelation in the iron also allows other important micro and macro nutrients to then be taken up also. So those billion parts of manganese are used as if you had dosed them also.