Is it true that the iron present in traces & chelated through EDTA process will be lost in case of exposure to light? A lot of people sugggests that the EDTA chelated iron should be dosed only after lights out.
Yes, it is lost from the water column due to photodegradation of the EDTA followed by precipitation of the Fe+3 (after oxidation from Fe+2). It's stilll in the aquarium - most likely in the gravel and filter.
The effect is most pronounced with wavelengths less than 400nm (UV) - thus the suggestion that a UV sterilizer not be run 24/7.
Fe+3 is a stronger chelator than some of the other trace elements needed by plants (like Co+2), so there is also a domino effect where the Fe+3 can "steal" an EDTA from a cobalt ion.
This paper: Laan et al, Environ. Chem. 2004, 1, 107-115. proposes a possible photostable, iron-binding-specific alternative to EDTA: desferrioxamine B. They showed that the compound is not photodegraded, but did not investigate the toxicity of DFO-B or its effect on the availability of the iron for plants (though they imply the data is being collected).
No, I don't wait until lights out. I've become quite the lazy aquarium keeper. Fortunately, my plants don't seem to care . . .
I dose my micros once a week after a water change - usually 3-4 hours before lights out. My lights are 6700K T8's, so not much emission below 400nm anyway - I'm also not "super high" light - only about 3WPG equivalent.
I also only dose my macros once a week - also after the water change (though I wait about 30min between the two) - like I said, kind of a lazy aquarium keeper.
FWIW, the article claims a half-life (time until half the Fe-EDTA complex is destroyed) of 6.1 hours under cool-white fluorescent tubes at 150 micro-Einsteins per square meter-second. I am not familiar enough with the units to speculate how this compares to aquarium lighting though.
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