Amazon Sword in declining health; diagnosis right?


We are having a problem with one of our amazon sword plants and 
I'm hoping someone can tell me if I'm on the right track.  The
plant is about 6 years old, about 14 inches side to side and 12 inches
base to crown.  The problem is that new leaves are coming in chlorotic,
gnarled and almost skeletonized.  After reading the FAQ on FINS, I am
thinking it fits the description of a trace element deficiency, but we 
do put Kent aquatic plant food in the tank with each water change (and 
have done for years).  My husband thinks it's just at the end
of its lifetime.

It is one of two swords in a 29gal high tank with one 
angelfish, three cherry barbs, two tetras, one SAE, and a trio of 
panda corydoras.  Water conditions are:  pH = 6.8, ammonia = 0,
hardness = 180 ppm, temp = 76F. Light is one 20W Triton (low, I know),
and the substrate is 3" deep fine gravel.  This tank has been
established now for 7 years.  The other swordplant in the tank is
younger, smaller (about 8 inches) and does not exhibit the problem.

If it is a trace element problem, what are we missing?  Could these
symptoms result from insufficient CO2?  Any suggestions/comments 
gratefully accepted.  Thanks!

PJ Jellison (jellison at ceps_nasm.edu)