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Homeowners Guide to Mosquitofish
(Gambusia affinis )

How to Obtain Mosquitofish

To obtain fish call the District office. Mosquitofish are normally available from April through October. Usually fish are delivered to the pond, but if you have a dog or keep the gates locked, leave a bucket of water on the porch and indicate the size of the pond when you call. The District is always glad to provide mosquitofish, free of charge to County residents, even on repeat calls, to encourage biological control of mosquitoes in ponds and other permanent water sources.


Mosquitofish play a very important role in mosquito control in Alameda County. Because of their great appetite for mosquito larvae, mosquitofish are very effective in preventing the production of mosquitoes from freshwater ponds and streams.

About the fish . . .

Mosquitofish live 2-3 years. They are live-bearing and produce 3-4 broods per year. The fry are often eaten by the larger fish unless provided with aquatic vegetation that is dense enough to offer protection.

Mosquitofish can tolerate a wide range of temperatures but may not survive winter in a shallow pond (less than 18"). The fish prefer the sunlit areas of the pond and do not thrive in a heavily shaded pond. At temperatures below 41 F, they move to the bottom, do not feed, and become inactive.

Mosquitofish will eat almost anything and one fish is capable of eating over 100 mosquito larvae per day. Feeding the fish is not necessary unless the pond is new and bare of vegetation. In this case, tropical fish flakes are suitable

New Ponds

Copper pipe or fittings in contact with the water can kill the fish. The pipes can be coated with a special paint available at hardware stores. Plastic piping is preferable.

New concrete ponds will leach lime into the water and make the water alkaline. A new pond should be appropriately seasoned (filled, allowed to stand several days, drained and refilled). The pH of the water is best in the range of 6.5 to 8.0. An inexpensive pH kit can be purchased at a pet or swimming pool supply store.

Wine or whiskey barrels will leach harmful chemicals into the water at first. They should be soaked and flushed out several times or lined before adding fish or plants.

The District supplies a handout on fishpond construction for your use in planning a pond.


Provide large rocks and vegetation for shelter from predators such as raccoons, possums, cats, herons and egrets. There should be rocks on the bottom in the deepest part, where the fish will spend cold days in an inactive state. At other times, since the fish tend to spend the night at the edges, overhanging banks serve well to help protect them.


This is a tiny floating plant that spreads quickly, covering the entire surface of the pond, especially when the water is polluted with rotting leaves or other organic debris. Fish usually do not survive these conditions. If the pond has a heavy coverage of duckweed, it should be cleaned, and the recurring duckweed kept to a minimum.


The green plant that coats the rocks and pond bottom is beneficial, producing 60% of the oxygen, and is found in a well-balanced pond.

Filamentous algae

This may indicate an excess of organic debris. If it gets too thick the fish may not be able to get to the mosquito larvae. Small amounts, however, are a good food source for the fish and shelter for the fry.

Unicellular algae

This turns the water green. It is not harmful to the fish but excessive amounts may indicate a high level of organic decomposition and a low level of oxygen. New ponds may turn green before becoming balanced, and an uncirculated pond will normally be somewhat green. Maintaining ornamental plants will help keep the water clear by competing for nutrients. Debris should be removed regularly. Use algaecides with caution, some are deadly to fish. Check at a garden nursery or tropical fish store for safe algaecides. Circulation and filtration are the best ways to keep a pond clear.


Many leaves, like pine, oak, eucalyptus and pittsoporum, contain chemicals that are harmful to fish. Accumulations of these leaves make the fish too sick to eat the mosquito larvae.

Mosquitofish Planting Policy

In an effort to minimize unwanted environmental impacts, mosquito abatement personnel do refrain from planting mosquitofish in sources known or thought to be habitats for endangered or threatened species. Care must be taken when planting mosquitofish in sources where they can migrate to habitats used by endangered or threatened species. Mosquitofish can still be planted in ornamental fish ponds and swimming pools in urban and suburban areas without worrying about endangered species conflicts. It is against California Department of Fish and Game regulations for private citizens to plant mosquitofish in waters of the State without a permit. (Title 14 CCR, Fish and Game Code, Section 1.63, Section 6400, and Section 238.5).

Mosquitofish provided by Alameda County Mosquito Abatement District are intended for mosquito control purposes only and should not be planted in potential mosquito sources by anyone other than certified mosquito control technicians or Fish and Game personnel.


Moquitofish plants that should not be a problem for
endangered species include:

Mosquitofish plants that require care to avoid conflicts
with endangered species include:

Please feel free to consult the District about planting fish