Re: beware of too much iron

On Sunday, 20 October 1996, Pat Bowerman wrote:

> As an experiment, I decided to tweak the PMDD
> formula with an extra teaspoonful of chelated iron. The
> result....nothing for about two weeks. Then, the plant growth in the 42g
> slowed dramatically, and soon algae started to reappear. Limnophila
> Sessiflora and Gymnocoronis Spilanthoides, which were being trimmed
> weekly, virtually stopped growing.

A couple of months ago, I did some investigation on how my UV
sterilizer was affecting my iron levels.  With the UV on, I started
increasing my daily PMDD to about 4x the usual levels in an attempt to
get my iron reading, which just barely registered, up to the first
mark on the Sera colour chart, 0.25ppm.  The results were dramatic.
The iron never did get to the target level, but some of the plants
started going rapidly downhill, particularly the wisteria; older
leaves developed lots of brown spots, then fell off.  Nitrates climbed
to about 40ppm, and cyanobacteria began appearing on leaves and
gravel.  The results weren't entirely negative; a few plants, notably
the giant duckweed and giant val, loved the new state of affairs.

Eventually I shut off the UV, and as the experts had predicted I was
able to maintain my iron levels with much less PMDD. (The process of
Fe(III) EDTA degradation is explained in a paper by Kodak.  Quoting

  It explains what the Fe (III) EDTA forms when exposed to sunlight and
  O2, but not the mechanism of initial attack.  The CH2COOH groups get
  pushed off the N atoms 1 at a time.)

The smaller daily dose of PMDD unfortunately lead to symptoms of
potassium deficiency, particularly bent leaves in the H. polysperma.
Cranking up the K2SO4 and KNO3 in the PMDD recipe solved this problem.
With the exception of the giant duckweed, which is growing much more
slowly, plant growth is once again exceptional.  It's actually a
little better than before; the Rotala macrandra is awesome now,
possibly due to the higher iron levels, and the plants bubble non-stop. 

Anyway, to make a short story long, watch out for those trace elements,
and only turn on the UV if there's an outbreak of disease in the tank.
Kevin Conlin  Montreal, Canada  "We're Canadians.  We HAVE to be polite"
Finger as332 at freenet_carleton.ca for PGP public key.