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Re: EDTA ( #594)

I must congratulate you on a succint summary. I believe it contains
everyting the average enthusiast needs to know. I wish my own
communication skills would improve.
Few plant enthusiasts attempt 'black-water' conditions, so I guess a
preference for Fe(2) chelates over Fe(3) has very minor consequence.For
the few who have high organic levels in their tanks, from peat extract
or whatever, the preference is much more significant. 
To put it as briefly as I am able:
Under acidic conditions your iron solubility is naturally much higher,
the problem with black waters is all the tannins etc.
Fe+3 salts will tend to react with your free organics (and other
oxidisable matter) rather quickly. A good proportion of this Iron will
then precippitate into complexes that EDTA and other chelates can't
redissolve readily. Ferrous:EDTA won't stay in solution as long as
ferric:EDTA. I use DTPA but the preference for Fe(2) is still very
important under my tank conditions.