Re: iron tests

Paul writes about the selectivity of iron test kits for Fe(III) state
I have a test kit from JBL that contains the thiocyanate sulfuric acid
mixture (I think - the test solution has a slight 'rotten egg' smell,
the colour change is towards pink and purple). The manual doesn't say
anything about selectivity.

Maybe some of you remember a discussion a while ago about my thread
algae problem. I was battling that stuff despite low (<0.05 mg/l)
readings on my iron test kit. I was using the Dupla trace element
tablets plus the daily drops. George suggested that my iron test kit
may be off and I reduced the daily drops further (five drops daily
only the second half of the week). I actually don't use any drops for
three days after a weekly water change, because the tablets also
contain enough iron, as George mentioned. Well, my iron test kit
readings are really low now (almost not detectable) and the thread
algae problem is nearly GONE. Now it's sufficient to remove them every
week or every other week. Plant growth is very good, no signs of iron
deficiency. I'm already thinking about increasing the light duration
from 8 hours mon-fri and 10 hours sat and sun to 10 hours every day.

Now back to the point: what do you all think about the idea that my
test kit is only measuring the most oxidized (III) iron, whereas the
chelated iron from the dupla products is in the (II) state? I had
always thought that the (II) iron gets oxidized rather quickly in the
aquarium (therefore the daily drops). Maybe that's not the case?