[Jeff Kropp]

Fertilator says I'm adding (three times weekly):
N 12.7 (ppm) (KNO3)
P 3.85 (ppm) (Monobasic P sulfate)

I think that addressing hardness factors is too far down the troubleshooting line for you. Consider this trouble shooting order: Light, C, N, P, K, Ca, Mg, Fe, then other micros.

You might take a look at your ratio of N to P. Currently your N/P ratio is 3.3. I have had trouble with low ratios causing N limitation when my frequency of supplementation outpaces the natural N production rates in my tank. My current lean supplementation uses a N/P ratio of 8.25, every other day, at 3 wpg. Tom Barr's estimative index recomends an N/P ratio of 10 based on a compositional dry weight analysis of aquatic vegetation.

Excess soluble P accelerates Fe reduction as does excess light.

Losing older leaves is often considered symptomatic of C and K shortfalls.

With a soil substrate, or an old and very mulmy substrate, you need to maintain good plant health. If a plant's respiration is too weak to maintain healthy roots in a strongly reductive substrate you will have trouble getting soil bound nutrients up into your plants.
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Jeff