As a fellow PPS-Pro doser I have often wondered the same thing.
I have a 20g grow out tank with a bunch of hygro in it and it seems if I don't add 3ml of csm-b solution each day the hydro begins showing signs of iron deficiency. Guess I really should get a test kit to see where I am at.
Will be following this thread to see how things turn out for you.
I am glad you brought it up.
Lately I see lots of threads about trace element deficiencies and toxicities. This uncertainty is going on for a decade now.
What is funny is that I set PPS-Classic with 0.01 ppm Fe and PPS-Pro with 0.1 ppm Fe. The reason behind this was to answer to people who were experiencing deficiencies. Personally I dose 0.01 Fe and donít see deficiencies. How come some people do and some donít?
Only recently I read on this forum a reasonable explanation. It was the substrate CEC Cation Exchange Capacity. This CEC explains why the discrepancies from one aquarium to another.
My aquariums have inert silica sand and that is probably why I am doing fine with 1/10th of the PPS-Pro Fe dose. So if we think about the other substrates, substrates with high CEC, we cannot be sure if there is deficiency or toxicity happening. Is the substrate new, saturated or old? Is the substrate absorbing, locking up or releasing? We donít know.
So back to the original question, ďHas anyone had an issue dosing PPS-Pro and still having low iron in the tank?Ē I have to say unlikely. Why? Because it is 10x more Fe then is sufficient with no CEC substrate. I think you are experiencing toxicity and therefore recommending 1/10th of the PPS-Pro dose.