Fluridone will knock out the cattails, and it's not a poison so perhaps that is an option. Unfortunately it is non specific, and will do a number on any beneficial plants also.

One of my former customers had a drinking water pond, and Fluridone was what he used every couple years.

"Forget pounds and ounces, I'm figuring displacement!"

If we accept that: MBG(+)FGSF(=)HBG(F1)
And we surmise that: BG(>)HBG(F1) while GSF(<)HBG(F1)
Would it hold true that: HBG(F1)(+)AM500(x)q.d.(=)1.5lbGRWT?
PB answer: It depends.