Black stems on Water Wisteria

[sesakaso]

I got some water wisteria and a red temple plant to put in my son's 20gal near the end of the tank's cycle. It did well until these last 4 or 5 days. The whole plant is about 12" tall with roots shooting out at every node. The roots all look healthy, but the bottom 4 inches of stem are now black. The black part goes past one node and up to the second one. There aren't many leaves there either, but the ones that are seem a darker green. The top is growing like crazy and is almost at the surface(needs to be trimmed). Before I do any trimming and replanting, should I get rid of the black stem parts or will they bounce back? What's wrong with it? The red temple hasn't changed in appearance(noticeably anyway) since I got it. I read those are slow growers, though. Oh, and the Java moss is settling in nicely taking over the decorative broken pot, just like I wanted it to. The shrimp are VERY appreciative! Thanks in advance for any input.

[SueD]

If the black part of the stem is mushy, it's probably rotting. This is an easy plant to just prune and replant the top portions. If the bottom portions are still healthy, they will also grow new stems. But if they've rotted, I would remove them and just plant the healthy top portions.