Anaerobic Substrate? What else?

In two of my aquariums that were NOT originally set up primarily for plants,
I've had some problems recently.

Background:  I had used just a gravel substrate, but after I got the plant bug,
I set up a (yeast/sugar) CO2 dispenser & began administering a few drops/day of
Kent Freshwater Plant supplement 7 added dupla-type "pellets" around the base
of selected plants; peat around others.  Have appropriate lighting (and then
some--get a couple of hours of afternoon sun).  I used only de-chlor drops &
bullseye 6.5 drops (for the fish) to tapwater; change approx 20-30% weekly,
vacuuming through top of gravel.  I use peat in the "whisper" filter.
Initial results:  I had to weed hygro & sags constantly, nymphaea were lush,
and the echino's were growing beautifully--out of the water & several inches
above the top of the aquarium (through opening at rear).   Anubias & crypts
were also doing wonderfully--everything did fine for over a year.  (Only a
giant Vallisneria seemed rather slow.)  Have always had just a small amount of
red (hair) algae w/o SAE's; otos & pleco keep the green algae pretty well
trimmed.  Duckweed got established, but I kept it trimmed as well.  No other

Problem:  It all stopped.  The echinodorus turned yellow; sags began dying off;
hygros became stunted & stopped growing. (tho' pieces of hygros that float seem
to stay healthy.)  Even the duckweed has disappeared, after first seeming to
yellow out.  In the aquarium w/o oto's, everything got covered with diatoms (I
believe that's what they are based on some exchange in this listserve).  Only
the anubias & crypts seem to still be somewhat healthy.  When vacuuming the
gravel, I get a strong, sulphury smell.

The fish all seem fine (dwarf rainbows, several types of tetras) & several
continue to spawn.  However, while I used to have a moderate pond snail
problem, that is no more; they all seem to have died out.

Any guidance as to what I should test for?  Will I simply have to break down
everything & start over, or can I try something else?  What might have gone