Tank Meltdown

Dear Plant People,

Help!  My gorgeous 20 gallon jungle has just experienced a meltdown of
apocalyptic proportions.  What was once worthy of pictures now looks like a
plant holding tank at a cheap pet store. Three dense stands of crypts melted
due to a change of reflector.  Messy, but at least I understand.  What I
don't understand is the subsequent deterioration of the H. polysperma and H.
difformis.  Both are rapidly losing their leaves leaving long, bare stems.
 The growth at the tips is very slow and the new leaves are tiny and
deformed.  For the first three months growth was very strong.  DIY CO2
injection produced vigorous photosynthesis.  By the beginning of the fourth
month everything seemed to grind to a halt.  The leaf tips of the val. spir.
are also yellowing, though there are many new runners.  What's going on here?

I suspect K deficiency, and that the plants were basically running on their
reserves until they ran out.  This deficiency probably didn't help my crypt
meltdown either.  I had been supplementing the tank with Tetra FloraPride
with Fe (3% K; 0.15% Fe) at half the recommended dosage, and more recently
PMDD at 2 ml per day.  I recently switched to the higher K/N PMDD formula,
though I am adding the KNO3 separately.

Tank specs:  20 gallons / laterite substrate / pH 7.0 / 12dGH / 12 dKH / 10
ppm NO3 / 45 watts full spectrum light (11 hours) / DIY CO2 / 4 platys, 5
rasboras, 2 SAEs, 1 Cory

1) Is my diagnosis sound?  
2) How much K2SO4 (potassium sulfate) can I safely add to the tank to test
this out?
3)  What else might I try?

Your help in restoring my tank to its former glory would be most appreciated.

Bill Cwirla
Hacienda Heights, CA