Black Stems

I'm a relative newcomer to planted tanks have focussed most of my energy
on my reef tank in the last few years.  I have a 15 gallon tank with 40
flourescent watts of light in a heavily planted.  The pH is about 6.8 in
the evenings, dkH about 3.0.  The plants are growing well except the
stems on the hygrophila polysperma (sunset) are turning black from the
bottom up.  The only supplement I added is Ferrovit.  The substrate is
plain gravel.  Also, I have a DIY co2 injector.  Are fluctations in pH
as tough on plants as they are on fish?