i'm reasonably new to the planted tank idea but i wonder if its from the uptake of the plants, and what he's listing is the running average. It would seem to me that it would spike up when added then drop off as the plants utilize them. I'm just not sure how fast the uptake is. From what i've seen on stem plants it looks to be pretty fast with good light and CO2.
Just my thoughts on the idea, interesting question though, i've only recently started reading the small print on his tanks and understand what it all leads too
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jason