If you acquire one of the comprehensive supplements, there is more than enough iron in them. Iron is a heavy metal, and thus highly toxic to all life forms. While we all need some iron, too much is deadly. It is one of the micro-nutrients for aquatic plants.
Plants take up iron, and can store it. If you start adding iron to create 1 ppm, you are undoubtedly overdosing the iron because the test can only measure what is in the water, not in the plants. I killed off the floating plants in one tank by using more iron than needed.
The whole point of a comprehensive supplement is that the nutrients are in a proportion to each other, according to the needs of aquatic plants as determined by scientific studies. The products I mentioned previously have this proportion so you cannot overdose any one of them.