I have tons water wisteria (hygrophilia difformis, I think) and that seems to be normal for it. When its small, it has little serrated leaves, but as it matures, it explodes into big lacy "fronds" at the top. After a while pieces of the fronds fall off and float around, growing roots on them, and the stem of the main plant dies from the bottom up. That is the strange lifecycle of water wisteria.
You can throw out the little rooted floaters, or plant them. You can also pick the live tops of the plants off of their rotten stems (just discard the stems), and replant the tops in the substrate, and they will continue to grow. Once I got used to this odd process, I learned to really like water wisteria. It is a beautiful background plant, and because it grows insanely fast, it's great for your water quality.