The pond doctor sold us something called Sonar. It killed it and never came back. That crap was so thick you could throw a rock out there and it would not even sink through the top layer. Its expensive and you have to ride around spraying it under water. It takes 3-4 weeks to kill it. Attacks from the root up, turns it pink, then brown, then it starts to sink and disappears for good. It has been 3 years and it has yet to come back.
Good Luck.
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