Discolored Ambulia?

I bought a beautiful large leaf Ambulia plant 16 days ago. Since then
the majority of the plant has turned a green/brown color while the
newest leaves and the tips of the old leaves remain a bright green. I
read these symptoms are either too little iron or too many phosphates
in the water. I don't have an iron test but all other plants are
thriving at the moment.

Ambulia is supposed to be a fast growing plant (right?) so I am
concerned this is an indication that will soon show in the rest of my
plants.

Any opinions?

55g-FW, 12 tropical fish, Asst Sword plants, Java fern, Ambulia
lights: 1)32W 5100K 1)32W 3500K 2)40W cool white tubes
substrate: soil (red clay) and sand with a layer of gravel
water parameters:~77F, 7.1pH, 7dKH(125ppm), 7dGH(125ppm)
0 ammonia, 5 nitrates, 0.75 phosphates

my aquarium page, info and pics at:
www.geocities.com/spiral_72/Spirals_page.html