the 'duckweed index' is genius for checking out whether the issue stems from CO2 V mineral deficiency.
I believe H Corymbosa needs a lot of CO2 and other nutrients because it's a fast grower given a light level.
I second the N/P/Mg leaf color / interveinal diagnosis.
Just a thought, could the brown spots be iron deposits in the leaves brought about by the .25pmm FE daily? I'd go easier on isolating the Fe and K needs of the plant, and concentrate on CO2 and ensuring that the other macro needs are met.
My personal view is to try and maintain a
constant but very low K concentration in the water, not more than 1ppm say, by small daily doses. High K makes no difference for most plants, but time and again I can induce calcium like deficiencies...curly new leaves on healthy plants such as L arcuata, R macranda, etc if I up the K. I know this observation gets shot down time and again in favour of not enough CO2 (particularly as the plant grows toward the light, and needs change, etc) but I'm taking about plants that were showing beautiful red well formed leaves (and now continue to do so) since going easier on the K...by no means a scientific observation I know, and I'm going a touch off topic.
I use tapwater solely, called "moderately hard" by water company, high Ca, really low Mg, High NO3 (20ppm), really high Sulphates (30-40ppm). So I make a daily mix of Magnesium Nitrate, Potassium Phosphate. Haven't yet seen a need to add any trace elements based on plant growth. The duckweed is looking good.
Good luck with the hygro!