One thing that Ms Walstad said in her book is that Fe+++ ions, the bioavaliable iron, in the water don't remain there except for a short time, because the plants very quickly take them out of the water. That is a clue, but I haven't figured how to use it. One possibility is that Phosphate ions remove the iron ions even faster than the plants can remove it. So, perhaps GSA needs more, not less, iron than other algae forms, so it is GSA that suffers the most when phosphate ions take away the iron ions. I'm not a chemist, even an amateur chemist, so I'm handicapped in thinking about this.