plants melting

Hello everyone,

Lately I've been having some trouble with my 10 gal. plant and fish tank.
But before I tell you the problem I will list all my tank conditions.  I
have had the tank set up for approximately 2  1/2 months since moving to a
new place.

temp.: 76-78 F
pH: 7.2
hardness: 120 ppm
buffering capacity:   80 ppm
light:  2  15 watt trilux bulbs (old)
filter:   small wet/dry  type canister filter
substrate:  med. sized gravel (size of frozen corn  ?? : )  )
inhabitants:  3 platys, 4 tetras, 2 cory cats, 1 otto, bacopa, amazon
swords, aponogetons, ludwigia,
                    and a cript, 2 malasian trumpet snails

For awhile my plants were doing well.  Usually they don't do much but this
time they actually appeared to be growing.  The bacopa was doing especally
well.  Then after a month I noticed that the ludwigia had lost most of
their original bottom leaves due to melting and one of my amazon sword
plants had started to rot in the center.  Now the bacopa doesn't  look so
well either.  The tips of the new growth have melted.  So now the only
plants that look well are the cript and the aponogetons which are
relatively new (1 month old).

I have been toying with the idea of adding DIY yeast CO2 for awhile.  Would
this be a bad time to start?  Or could it be what my plants need.  I
thought perhaps they needed more trace elements which is why I'm using
Flourish but it doesn't seem to help either, maybe I've added to much.
Without a test kit, are there any symptoms.   I've tried to get a test kit
for iron but my local store didn't even know one existed.  Ditto for
laterite.  If you have any ideas I'm open to suggestions.  Oh and if you
know what chemicals are used in the test for iron please tell me, I would
like to try to make my own.

Thank you in advance for any advice and for listening to my rambling,

Cindy  (South Bend, IN)