Emmersed plant deficiency????

Hi all,
I'm at a loss as to what can be causing my emmersed E.cordifolious to be yellowing just on the edges of the leaves in my puddle tank. The substrate is very nutrient rich, ada new amazonia, and also has Tom barrs low tech dosing once a week with almost no water changes. No other plants above or below the surface are doing this. Any ideas anyone???

Here's a pic


Any help would be greatly appreciated. Thanks