I won't dispute Greg's suggestion (REWARD) - especially for fast results; but my preference for APW (Potamogeton nodosus) is AQUATHOL K (endothall, liquid) applied topically at a relatively high concentration (ie. using a low ratio of water-carrier).
Although REWARD yields a very quick brown-out, rooted plants often return (at least in our part of the country).
AQUATHOL K has a different mode-of-action that somewhat suffocates susceptible plants - usually killing them root-n-all.
One advantage for REWARD is the ability to greatly limit the area of impact to that which is physically treated, while AQUATHOL's impact could migrate into untreated adjacent areas.
