Jason, I agree with Shan, but Navigate is also an excellent product for certain situations. It is ideal for spot treating Eurasian Watermilfoil and Coontail at rates of 100-200 pounds per surface acre, and since it is granular, you can just spread it over the areas you want to control.
Duckweed is a tougher nut to crack. If you tank mix 1 part Cutrine Plus & 2 parts Reward, plus non-ionic surfactant and apply at the rate of 1-2 gallons per surface acre, you will get some control of both surface algae and duckweed.
You may want to think about some preventative measures to cut down on the chemicals. If possible, prevent run-off from directly entering your pond. We recommend a buffer zone around ponds composed of deep rooted native plants, like sedges. This creates permeable soil that will soak up some of the run-off. Also, do not apply lawn fertilizers, or at least use zero phosphate brands. Aerate to help the pond process the nutrients. Some types of aeration can also help with surface algae & duckweed. If possible, rake out mats of algae and weeds to cut down on the amount of organic matter in the pond. Establish good native vegetation in and around the pond. Examples are water lilies, bulrush, wild celery (aka eelgrass). Don't kill off all your Coontail, because this is a native weed that helps the pond process its nutrients.
Those are just a few things you can do to cut down on nuisance weeds and algae. They may not all be applicable to your situation, but they should give you an idea of how to approach your problem.
