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Questions about PMDD

I have a few questions about PMDD that I hope someone could answer.  Let 
me start out by stating that I won't argue about whether PMDD method 
works.  It clearly does.  I am having trouble determining proper dosing.  
If I understand correctly the best method is to measure the iron content 
of the water and dose enough PMDD to keep iron levels at .1 ppm.  I'm 
using terralit as a substrate additive and have never had insufficient 
iron level in my tank <.1 ppm (also no signs of iron deficiency in my 
plants).  In addition recent posts have been pointing out that the test 
kits used are not measuring chelated iron.  The forms of iron 
registering on the tests are not the ones being used by plants.  
    This leads me to think that I should be using another method to 
determine dose size.  Plant condition might be a option but I have never 
observed an overt element deficiency in my plants.  Just slow growth in 
some species, rising nitrate levels, and outbreaks of algae. Is there 
another trace element I can measure effectively to determine PMDD dose 
size.  Ideally it would be something used by the plants readily and 
quickly maybe sulphur (.05% to 1.5% concentration in dry weight if 
plant) or manganese (.0005% to .15% concentration in dry weight of 
plants).  How quickly is sulphur and manganese used in plants, and can 
they be measured effectively.

Any input anyone can give would be greatly appreciated.

Thank You,

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