CUSO4 is a viable product in the proper situation and if used correctly. It may not be the best product and it probably is not the right one in your situation. But it depends on the plants and the situation. You have to id the plants and the control method. See this link to help.
http://aquaplant.tamu.edu/ This assumes you have fish and don't want to risk killing them or damaging the pond dirt and water. If that is not the situation you can drain the pond and renovate to get rid of the weeds.
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