I have used........... I mean read somewhere that a product called Karmex (which used to be labled for ponds) might work well.
Otherwise rodeo will do a great job.
There is also a chemical called Eagre we sell a lot of for a pnd herbicide, although I have not tried it myself.
The important thing to remember about any aquatic herb is that if you kill more than a third of the pond at one time you run the risk of killing fish from a lack of oxygen. So any aquatic herb can kill fish.