How long have you had this plant so far? It will probably die with time, or otherwise won't do well in the long run. I mean, I wish you best of luck with it and I hope it will grow very well for you. I am just telling you what I have read about this plant. If the plant gets too high, you can cut it off at the top. Here is the information about cabombas and their propagation:
http://plantgeek.net/plantguide_sear...ba&type=Search I don't know what kind of cabomba you have, so here are a few types. Each is described.