Plants not doing nearly as well as they used to

Hello everyone,

During the first year of my 40-gallon planted aquarium, my plants looked
great. In particular, I had a huge, beautiful Echinodorus. My other plants
were also thriving. In fact, at times I could actually see streams of tiny
bubbles coming out of them. I assume it was oxygen, which I took as a sign
of good health.

Unfortunately, they haven't done this in a long time, and they don't look
nearly as good as before (that was maybe two years ago). If the bubbles were
indeed a sign that the plants were thriving and producing great amounts of
oxygen, what could explain their current situation?

What puzzles me is that I'm actually taking better care of my plants for the
last year or so. I've been adding fertilizer on a regular basis (but not too
often or too much), and I recently started adding CO2. I've also always made
sure I changed my fluorescent tube often enough so the light quality
wouldn't go down too much. Ok, so I only have one main light for a 40-gallon
aquarium with a large surface area, which isn't enough, but the situation
was the same when my plants were doing better.

Is there anything that could explain my plants' decreased health under
improved conditions? My old Echinodorus was 4 times the size of those I have
now, and it didn't tend to turn yellow or brown or lose leaves like the
current ones do. The situation is similar for other species. Could it be
that I now have too many plants? I'm pretty sure I have more now than 2
years ago. Are they competing with each other or something?

I realize that it could be any number of things, but any advice would be
greatly appreciated. Thank you.

Francois