Anacharis/ elodea - looking rather sick

["Kinney, Travis" <kinney at pdtarchs_com>]

Some of my plants, especially the elodea, isn't looking very good. When I
bought it, it was a nice dark green. It is growing fine, but the leaves are
a light olive and kind of transparent. I noticed a lot of my plants lack the
deep green color of when I bought them. Sword grass, anubias, riccia, and
even my duckweed, all seem to be a pale greenish yellow. Is there a certain
mineral that a plant needs in order to keeps it a deeper green color?