Cattails aren't all bad - they are pretty good at controlling shoreline erosion. IMHO killing all of them at once does not greatly impact water quality (like wiping out a pond full of FA can). But, you may get better contrary info on that from others.

Glyphosate (the active ingredient in Roundup) will kill cattails. There is an aquatic formulation which has been mentioned here, but the time I used it I used plain old Roundup. It is reported to be hard on amphibians. I sprayed from my jon boat so that the overspray was on shore, not in the pond. I still had four million (estimated) bullfrogs and tadpoles in the pond afterwards, so that may have been a good approach.

When I have only had a few cattails, I pull them, which decreased their numbers enough that other shoreline vegetation took over and has prevented their recurrence.

"Live like you'll die tomorrow, but manage your grass like you'll live forever."
-S. M. Stirling
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