- 1). Fill a nylon mesh bag that has a drawstring closure with barley straw. You can purchase barley straw in bales online from garden pond supply stores. Place a piece of polystyrene foam inside the bag to help it float, and weight down the strings of the sack with a brick. Sink the straw into the pond below the aquatic plants. As the straw decomposes, it releases chemicals that kill algae and invasive plants. According to Utah State University, one nylon bag per 10 square surface feet of pond is enough to kill aquatic weeds and algae. Barley straw should be changed every three months to effectively keep plants under control.
- 2). Place fish such as koi into the pond as an additional measure or as an alternative to barley straw. The presence of the barley straw will not harm fish. The fish will eat invasive plants such as duckweed, which will help to keep the plant population down. According to the University of Ohio, good control of duckweed is maintained if you add koi at a rate of 50 fish per acre. Koi should be placed in a pond in spring prior to duckweed budding, when the plant has yet to cover the pond. They will not effectively control duckweed if added later, due to the fishes' inability to out-eat the duckweed budding rate. Leave the fish in the pond after the plant population is under control. The fish will not reproduce at rates that overpopulate the pond.
- 3). Pour a copper sulfate-based chemical herbicide such as bluestone in the pond. Copper sulfide is a natural mineral that is harmful to aquatic plants. When used as directed, this herbicide will not harm fish.
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