Hey John,
I got an idea for you.... If you want to try the cattails fine, I won't discourage you. Here is what I typically advise my customers to do. If you are going to introduce cattails then place them on the side where the wind won't blow the seed into the pond. For example: if the wind generally comes out of the West then plant them in the Eastern part of your pond. If you want to somewhat control them, make sure you cut all the seed pods off as soon as they come up.
Worst case scenario us Rodeo and a surfactant to kill them. Only professionals should use a chemical like Habitat. It has a residual of up to two years in the soil. Not a big deal unless you have trees around the pond, which it will kill everyone!
