Skip to main content


The District will deliver fish, at no additional cost, to San Mateo County residents for control of mosquitoes in backyard ponds. The water in the pond must first be treated to remove chloramine (a sterilizing agent that is present in tap water). Products to remove chloramine can be purchased at a local pet store or aquarium specialist.

It is against California Department of Fish and Game regulation 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). Do not introduce mosquitofish to rivers, stock ponds, lakes, or creeks.

What are mosquitofish? 

Mosquitofish (Gambusia affinis) are small fish in the guppy family, which are used extensively throughout the world by mosquito control agencies for control of mosquito larvae. A single adult mosquitofish can eat up to 500 mosquito larvae a day. The adaptability and hardiness of the mosquitofish, coupled with its ability to produce large numbers of fry (newly born fish) during their lifetime, has made them a valuable biological control agent. Female mosquitofish are 2 to 2 ½ inches in length, while the males seldom exceed 1 ½ inches. These fish are live bearers, producing 60 to 100 fry in a single brood. Under favorable conditions, the fry reach sexual maturity in six to eight weeks. Females may bear three to four broods in a season, the first of which may number only a few fry. Reproductive efficiency declines with age and later broods become smaller.

What does San Mateo County Mosquito and Vector Control District do? 

The District responsibly employs mosquitofish only in situations where they cannot escape into natural bodies of water, such as suitably contained backyard ponds, ornamental pools, fountains, and neglected swimming pools. This service is done at no additional cost to residents. If you have a pond and would like to have fish or request a pond inspection, please call the District Office at 650-344-8592. Mosquitofish are not allowed to be used in natural sources where they can reach creeks or other natural waterways.

Tap Water, Chloramine, and Mosquitofish 


Tap water in San Mateo County is now treated with chloramine, rather than chlorine, as it was in the past. Chloramine is toxic to fish, shellfish, reptiles and amphibians, but does not affect humans or mosquitoes.

Chloramine is a chemical containing chlorine and ammonia. Chloramine is considered safer for drinking water than chlorine because it reduces the formation of hazardous by-products and lasts much longer than simple chlorine. Unlike chlorine, chloramine does not evaporate from water if it is left standing overnight.

Water used for fish must be treated to remove both the chlorine and ammonia components of chloramine. This can be done by running the water through a properly sized activated charcoal filter, biological filter or by treating it with chemicals. Chemicals designed to remove chloramine are also available at pet stores, fish supply stores, and some variety stores that sell pet supplies. To be completely safe, always pre-treat your water before adding it to your pond, no matter how little you add. Replacing as little 1% of the water in a pond with fresh tap water will add enough chloramine to kill the fish. Chloramine residuals in treated water should be below 0.1 mg per liter. Kits to test the amount of chloramine in the water can be purchased at most pet and fish supply stores.

For additional information, contact:

  • San Mateo County Water Quality, (650) 363-4305
  • State Water Resources Control Board, (916) 341-5250
  • Environmental Protection Agency Region 9, (415) 941-8000
  • San Francisco Public Utilities Commission, (877) 737-8297
How many mosquitofish do I need? 

Normally 6 to 10 fish are sufficient to stock an average sized pond. The high reproductive potential of the mosquitofish will overcome future problems with mosquito larvae. Where a large number of mosquito larvae exist, fish may be insufficient in controlling them. In such cases, it may be necessary to clean the pond or to have the District treat it with environmentally safe insecticides prior to planting mosquito fish.

Transporting fish 

The method of transporting mosquitofish is critical to their survival. Glass and metal containers rapidly conduct heat and in some cases may be toxic to the fish being transported. Avoid sealing the container, unless the air space is equivalent to the water space in the container. Large plastic containers, those with large surface areas, are the most practical. Keep the containers out of sunlight during transport. Fish should be introduced into the pond as soon as possible. The carrying container can be placed upright in the pond, for 30 minutes to an hour before releasing the fish to allow the two water temperatures to equalize.

Do I need to feed the mosquitofish? 

Probably not. Mosquitofish in an outdoor pond do not need be fed during the warmer part of the year. Mosquitofish can survive on the insects and algae that are present. During the colder months, the fish become lethargic and feeding activity is reduced. Commercial flaked fish food may be used as a dietary supplement when mosquito larvae are not present in the water. Supplementary feeding should be done with caution. Artificially fed fish make poor mosquito predators. Overfeeding can also cause the water to become fouled, which can be lethal to the fish.

Optimal Habitat 


  • Treat all tap water with a chloramine removal kit
  • Ponds should be at least two feet deep
  • Pond edges should have steep sides
  • Rocks and aquatic vegetation should be present
  • Provide a circulation pump to enhance water quality
  • Pond lining: If it is a new cement pond, allow several weeks to leach out the lime. Rinse the pond several times and fill with fresh water. After 48 hours, request mosquito fish for pond.


  • Thin or remove excess aquatic plants and keep landscape plants trimmed away from the pond edge
  • Fallen leaves can be toxic to fish; remove dead vegetation frequently
  • Avoid shallow, sloping edges

Shade vs. Sun

Although mosquitofish prefer the shelter of rocks, overhanging plants, and banks, they do not thrive in heavily shaded ponds. Normally, the fish seek out a sunny section of a pond and orient themselves to shallow areas with submerged vegetation, locations often frequented by mosquito larvae and pupae. It is recommended that some aquatic vegetation be provided in ponds as shelter since mosquito fish are cannibalistic and will feed on smaller fish. In large ponds, the mosquitofish will sometimes avoid the shaded areas even though numerous mosquito larvae are present. This may occur if the fish are fed artificially or if there is an abundance of naturally occurring food in other areas of the pond.


Limited amounts of algae can be beneficial to mosquitofish. An overabundance of algae should be removed. Removal can be achieved by physically cleaning the pond or application of an appropriate algaecide. If an algaecide is selected, care must be practiced when following the directions. An overdose of the chemical could be fatal to the fish.

Should I put mosquitofish in my bird bath? 

Bird baths or plastic wading pools are not suitable as fish ponds. Shallow water does not provide adequate protection from elevated temperatures or predators. In addition, high temperatures enhance algal growth and deplete oxygen in the water.

Do I need to get new fish every spring? 

Mosquitofish are fairly hardy and can live under a variety of conditions. In California, mosquitofish can live for two to three years. They can often live through the winter in outdoor ponds, but at very low temperatures (below freezing), they may not survive.


Join our mailing list