Sonar is an excellent choice. It isn't a contact killer, and must stay in contact with plants for a long time, up to 90 days, to allow proper uptake by the plant. With lots of water leaving, Sonar's efficiency will likely need to be boosted by additional treatments. Our best advice is to talk to the distributor where you intend to buy the product, have them put you in touch directly with a SePro rep, who will design a protocol specifically for your pond.

Teach a man to grow fish...
He can teach to catch fish...