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how to dose Fe

I'm reading problems of folks having to cope with inaccurate Fe test
kits and trying to guess their iron concentrations. Here is another
suggestion and I hope someone more familiar with the chemistry than
myself will follow this up.

Assume that in a brightly lit aquarium, all the Fe-EDTA+DPTA will be
oxidized or consumed in one week. Somebody with a highly accurate test
kit could come up with some better concentration loss factors in high
and low light conditions. Assume well aerated conditions where the
oxygen concentration is close to equilibrium with the atmosphere (well
aerated, no plants). Calculate the amount of chelated Fe trace element
mix needed in grams to produce 0.1 ppm for X gals of water where X is
the number of gallons of water in the tank. Divide this number by seven
and this should be a safe daily dosage which will not exceed 0.1 ppm.
BTW, iron loss is probably not linear with time but proportional to Fe
concentration. So what we actually want to measure to get a better value
will be the % loss in Fe for one day as a function of light intensity
and temperature (assuming that lighting and temperature are the dominant
parameters for chelate decomposition).

You could also use the same method, dosing a known volume of water with
a known concentration to calibrate your test kit. You should calibrate
it both for the expected color development time and for a longer
development time to see which might be more accurate. The problem might
be that the chelate is interfering with the chemical reagents in the
test kit and hence the long development time may be necessary (or that
the test kit is simply not appropriate for Fe-EDTA+DPTA which is the
most stable chelate)