Most counties, such as the counties of Santa Clara, Los Angeles and San Diego, distribute mosquitofish for free through the local vector-control program. Depending on the county, some vector-control districts even deliver them to the residents without charge.
Mosquitofish have long been used to effectively control mosquitoes in areas where there is an abundance of mosquito larvae. The fish stay and swim close to the water surface where the larvae are deposited, eating them and preventing them from reaching their pupal and adult stage.
Unlike other fish, the mosquitofish do not lay eggs, but produce live young. They do not require a special environment to protect their eggs, and the young ones can feed by themselves. Mosquitofish also reproduce quickly, producing 50 to 100 broods in an interval of six weeks. In some counties, such as San Diego County, if a resident has too many mosquitofish, the vector-control district can come and collect the extra fish.
The mosquitofish may compete with other fish for food and feed on other live things in its habitat. Because they are prolific, they can easily outnumber other types of fish. They are used primarily in controlling the mosquito population in green swimming pools, ornamental ponds and fountains. In most states, residents are not allowed to plant mosquitofish in state-owned waters, such as rivers and lakes.