Well, I went ahead and removed the sick plant and the two smallest plants leaving just three plants in the grow. The roots and all of the plants look very healthy. The only interesting thing of note is that the roots of the sick plant had a very different growth pattern from all the other plants. All of the healthy plants have roots that are about 15" long - reaching deep into the reservoir - but relatively narrow. The sick plant's root mass was only about six inches long, but about 8 inches wide. Not sure if that means anything, but it was interesting.

I also moved the remaining three plants around to give them more room.

Thanks for all the help.