Duckweeds are many small, floating water plants that live in nutrient-rich aquatic habitats. These plants can grow vigorously in environments contaminated with fertiliser, or where fresh water flow is reduced. Duckweeds are efficient at removing nutrients from waste water and are desirable food for some fish and birds. However, when these plants overgrow, they can cause environmental problems.
Grass carp, also called white Amur and Ctenopharyngodon idella, are plant-eating fish native to Russia and China's Amur River basin. These fish survive in a range of environments, including ponds and streams, and can tolerate water temperatures between 1.11 to 35 degrees Celsius. In the United States, these fish are often used to control duckweed infestations. The United States Department of Agriculture recommends choosing only triploid grass carp for this purpose. These fish are sterile and cannot reproduce. Ordinary (diploid) grass carp can quickly overbreed and become invasive.
Tilapia are herbivorous fish originally from Africa and the Middle East. These freshwater fish are often bred for food and will eat a wide range of aquatic plants, including duckweed. They survive only in relatively warm water and do best between 77 and 86 degrees Fahrenheit. Tilapia tolerate partially saline environments and have become invasive in Texas, Florida, Alabama and Southern California. They can be used for duckweed control in enclosed ponds but should not be released into the wild. Captive farmed populations are sometimes fed duckweed for increased growth.
Giant gourami, or Osphronemus goramy, is a medium-sized fish originally native to southeast Asia, where it is often grown for food. This fish eats duckweed and other aquatic plants, as well as small invertebrates. It can live in fresh or brackish water and does particularly well in slow-moving waters. In Asia, gourami grown for food are often fed on duckweed. This fish has become invasive in Florida, Hawaii and Oregon.
The channel catfish, or Ictalurus punctatus, is a North American native fish that lives from southern Canada south to northeastern Mexico, and from the East Coast west to the Rocky Mountains. They are popular sport fish and can weight up to 26.3kg. They consume insects, mollusks, crustaceans and other fish, as well as plant material like duckweed. In captivity, these fish are often raised on an entirely vegetarian diet. Channel catfish are extremely hardy and do best in large streams or small rivers where the current is low to moderate. They can be used to control duckweed populations without concern of invasiveness, but may alter pond and stream ecosystems.