Cattails are actually used in areas that are heavily polluted to not only remove nitrates, but heavy metals. Some of the parks in Los Angeles have small man made lakes that used to be so polluted that they would not support fish, and water fowl were getting sick. Now with the lakes partially bordered with cattails, and with man made floating rafts of cattails the lakes are able to support fish again, and they are finding the floating rafts are like fish nurseries!

They can grow very fast, but if kept under control they have multitudes of benefits!

http://en.wikipedia.org/wiki/Typha

Cattails are edible too!

http://www.articlesbase.com/camping-articles/the-cattail-best-of-the-wild-edible-plants-495731.html