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Elodea began to disintegrate or "rot", please advise...

Hello all,

I'm having a problem in my 29 gal. tank.

The elodea, which normally grows very fast, between 1/4" and 3/4" per day,
suddenly begins to disintegrate partway up the stalks.
Symptoms are a lack of chlorophyll, the stem and leaves become translucent
and all green goes away over the course of a few days. The growing tips of
the plants remain green but with reduced growth, a very short inter-nodal
length and the new leaves are "clamped" against the stalk.
After a few days the stem comletely dies in the area where the problem
began. Next the death spreads down the stalk toward the roots or base. Until
this point the lower portions of the plant usually remain green and appear

I can't seem to match these symptoms with an obvious deficiency, maybe I'm
reading too much into the deficiency descriptions and it's something
blindingly obvious?

What adds to my confusion is that the other plants are doing VERY well.
Several sword types have excellent color and are putting out new growth. The
bronze crypts are growing well & spreading. The Hygrophila polysperma is
doing better than it was recently, with better leaf size and growth rate.
The corkscrew val. is still spreading via runners. The val. however is
showing some yellowing on older leaves but they might just be old--I'm not
sure if this is related or not. Java moss is "reclaming" sections of the
tank at a decent rate.

Any thoughts? Why is this one plant species rotting away?

Tank parameters:

Temp 74 deg. F
Flourite substrate with minor amount of gravel & clay present lots of mulm
in substrate.
pH 7.0
KH = 2 deg. (this is adjusted with baking soda during water changes, tap
water tests between 0 and 1 deg)
GH = 4 deg.
Ammonia = 0
NO2 = 0
NO3 = ? somewhere between 2.5 and 10 ppm depending on the test used.
    This value is problematic because the two tests (liquid only reagents
vs. powder reagent) both seem to indicate a clear value. Perhaps I should
take the "middle" value on the chart and say it's 5 ppm? I know this value
is important but the tests are frustrating. There have been several threads
posted recently regarding the accuracy of these tests and how to read the
results properly. (sigh) Water out of the tap shows 0 for both tests.
Nitrate is added via KNO3 in homemade PMDD.
Phosphates = ~.25 ppm.
Iron = .1 ppm. Iron is added via trace element mix. (Flourish trace)

Light is a whopping 3.7 watts/gal via a dual compact flourescent fixture.

CO2 has only been running on the tank for 2 days via DIY method. The problem
described with the elodea began before the CO2 was added. I'm not sure how
much exactly is there. The "chart" or calculator suggests 6 ppm. (bubble
rate is currently 1 bubble about every 4-6 seconds) Bubbles get injected
into the input of a powerhead and come out in a cloud of tiny bubbles. Some
of the gas never dissolves as the bubbles float to the surface.

Any general observations would be very much appreciated and any insight into
the main question:

Why the heck is my elodea disintegrating?