Folks ask me how much I dose, it's often quite a bit.......except for NO3 and light really, but that's still 15ppm 2-3x a week also.
CO2, most of the rest are quite high.
If you plug and chug, I add about 2-4ppm a week typically.
The ONLY research study on submersed plants and Fe I've ever found was on Hydrilla and it had a max growth rate at 6ppm and a max uptake at 8ppm.
0.1ppm comes from the old old days of PMDD....when folks used 2-5x less light.........50% to 20% of typical CO2 ppm's...........and thought Fe excess caused algae
And then a couple of methods copied that amount lock stock and barrel and did not test things themselves. Or worse, if they did test, they did a bat guano poor job at it.
The research ref is from Haller 1977.
http://www.sciencedirect.com/science/article/pii/0304377077900390
Another concerning translocation from the leaves to roots and vice versa:
http://www.springerlink.com/content/j314607751762514/
NEVER once has anyone cited either of these in 15+ years in this hobby other than myself. Not once.......
But what do I know?