Holes in H. polysperma/difformis

Good people,
May the flora and fauna of your tanks flourish in the New Year.  

It is now month #4 of my 20 gallon densely planted tank and the questions
begin.  I have recently noticed that the lower leaves of h. polysperma and h.
difformis are turning a mottled yellow/yellow-green and are full of tiny
holes.  The leaves of h. polysperma will eventually turn completely yellow
with many holes, though the leaf remains firm, not mushy.  The h. difformis
develops yellow blotches that eventually die away, leaving the leaf looking
as if it were munched, though I doubt that this is being caused by fish or
snails.  The photosynthetic activity, which was once quite vigorous, seems to
have slowed considerably in the last few weeks - very few bubbles on the
underside of the leaves.  The upper growth on each of the hygro stems appears
normal, however.  The other plants (crypts, aponogetons, val) are doing well-
growing and reproducing.  No significant algae to speak of.

Tank parameters:  20 gallons; 3x15 watt full spectrum lighting (11 hours);
laterite-treated substrate; DIY CO2; pH 7.1-7.2; 13dKH; 11 dGH; NO3 - always
< 10 ppm, usually not detectible

Maintainence routine - 2.5 gal water change weekly; 2 ml Tetra FloraPride
w/Fe added every other day or so; Tetra fertilizer tablets pushed into gravel
occasionally around crypts and aponogetons.

The only change in the routine over the last month was to use 50% RO
water/50% tap water for the water changes instead of 100% tap water.  (Tap
water here is pH 7.8; 12dKH; 11 dGH; 10 ppm NO3).

I'm thinking either not enough light, though the light seemed sufficient a
couple of weeks ago (bulbs are 3 months old), or, the vigorous growth of the
hygro combined with the use of 50% RO water has depleted some nutrients,
macro or micro.  Any suggestions from your experience before I start

I am most appreciative for your advice and the ongoing collective wisdom of
this group.  This on-line digest adds an entirely new dimension to an already
fascinating hobby.

Happy New Year!

Bill Cwirla
Hacienda Heights, CA