Zosterifolia : Leaves have dark patches and some transparent

I just planted some H. Zosterifolia last weekend around the time I injected
2 bubbles/sec of CO2
The rest of the older plant residents: E. Stelata & Hygro Difformis are
doing fine. My Stelata actually
bushes out at the tip, looking like a beautiful ancient plant. My Difformis
leaves turn vibrant dark green
after I dosed the tank with Tetra Flora Pride and Seachem Potassium. Now my
Zosterifolia is not doing
very well. The older leaves seem have dark line patches towards the tip.
Some of them are transparent
with blackening. Zosterifolia can withstand up to 28C which my tank is at
daily. I wonder what else is missing to
stop the darkening of the leaves. Or is this a case of CO2 shock where the
mobile nutrients are sucked up
to the new leaves?
gH 6dH
kH approx 3
pH 6.6
Lighting at 3.5 watts/gallon

There are about 20-22 Tetras, Lampeye killies and Rasbora Espei, 4
Corys and a Pearl Gourami in this 15 gallon tank.