Ambulia problems

I have some ambulia in a well planted 55 gallon tank. The lighting is
approximately 4x40w daylight tubes and the substrate is a topsoil/vermiculite
mix covered by gravel. CO2 fertilization is by a canister. The ambulia seems
to be growing fairly well, but alot of it, especially the lower-down leaves
of the plant, become predominately brown after a time and start to slowly
wilt and ,eventually, fall off (the stems' color seems unaffected). When I
cut the plants back, the new growth shows no sign of this problem (being the
attractive light green). This is true even if the new growth is coming out of
a stem with leaves that are showing brown. It can be growing near the top of
the tank, near the bottom, or a small clipping that I have replanted and
there are no problems, as long as it is new growth. Eventually however, the
brown coloration starts appearing again (except on the the growth up at the
surface of the water, nearest the light. Which is also of course, the newest
growth). I've briefly tried rubbing my fingers on the brownish parts, and
nothing seems to come off, which is not what I would expect if it was
something like diatoms. In any event, I would think the lighting is strong
enough so that diatoms would not be a problem. I know some plants lose lower
leaves as they get taller or if crowded together, but this problem extends
too far up the stem too be explained by that. Could it be some kind of
nutrient defficiency? I don't add anything except dechlor to the water, but I
do weekly 33% water changes and am told that my local tap water is very
mineral rich. I have 23 fish in the tank (mainly tetras and otos), and feed
them once a day. This problem doesn't seem to be affecting any of the other
plants in the tank (Amazon sword, pygmy sword, java moss, dwarf swordplant,
red ludwigia, rotala macandra, bacopa, water sprite and vals). Has anyone
else had any experience with this, or any thoughts about it?

                                 Thanks for any help,

                                                Matt MacGregor