Long ago I found i needed more iron for things to look good, and upon some guidance about Ph and Seachem iron, I switched to 11% DTPA mixed.
I am adding iron daily at a rate of about .4ppm (specifically as an example in my 40G I add 5ml of a mix of 54g 11% DTPA FE and 500ml of water). My tanks run about 7.6-7.8 ph, dkh=4-5, dgh=7 @ 78F and with about 9 hours of low to almost medium light.
By the next day it is almost all gone. I am testing with the Seachem Iron test, waiting the 45 minutes suggested, and I get a trace in the low-range test (I'd estimate about 0.01 - 0.02 at most). I have tested the test with the included iron reference sample and it worked fine. If I test immediately after dosing I get approximately the expected value (maybe .2-.3 instead of .4, but a substantially higher reading in the right ballpark).
The tanks are low tech (though i do use Excel), and moderately planted. I've included a recent photo of each to give some feel for density since "moderately" is subjective.
How should iron behave? Does such rapid dissipation indicate that I am not dosing enough? Can this density of plants actually absorb that much, so quickly? Or is it precipitating out or otherwise disappearing for non-nutrient reasons?
I am trying to dose for a more steady state environment with infrequent water changes (say every 30 days), so I do not want to over-do it, at least substantially so.
Should I just keep increasing until after 24 hours I still have, say, 0.1ppm or so?
PS. I do use Osmecote+ root tabs in addition.
PPS. For reference, as I read it, the PPS-Pro recommendations are for about one quarter this much iron daily.