Mosquito Fish (Gambusia affinis) | Ultimate Care Guide

The western mosquito fish (Gambusia affinis), commonly referred to as western mosquitofish, western mosquito fish, or just the mosquito fish, is a species of small freshwater fish, originally native to North America. Its natural habitat was the streams and rivers of Indiana and Illinois, as well as the Mississippi River and its tributaries, all the way south to Mexico. They are now found on almost every continent on the globe.

Mosquito fish are small and dull grey in color, with faint hints of yellow and blue. They have a large abdomen, rounded dorsal and caudal fins, and an upturned mouth. Mosquito fish are capable of changing their colors slightly in order to match their surrounding environment. Due to the fact that they’ve been introduced all over the planet, they are ranked as one of the most invasive species in the world. They are also a very hardy fish, able to withstand temperatures ranging from freezing all the way to 100°F.

While they don’t usually eat adult mosquitos, mosquito fish do eat the larvae of mosquitos. A female mosquito fish can eat 150% of her body weight in larvae in a day, which can equal hundreds of larvae. For this reason, they have been introduced all over the world as a way to control mosquito populations. While the effectiveness of this form of biological control has been debated over the years, they are still being introduced into new environments to attempt to control mosquito populations before they’re able to leave the larvae stage.

Mosquito Fish Facts

  • In their natural habitats, some of their natural predators are catfish, bluegill, and bass, as well as birds such as egrets and herons, and raccoons
  • Female mosquito fish can store milt from multiple males at a time, for later use. She can use this stored milt for several months to have as many as 2 to 6 more broods. Generally, each brood will be smaller than the last one
  • If the female mosquito fish’s gestation period is a bit short, and the fry aren’t fully formed, they will be born with a yolk sac connected to them. It will be connected through a slit on the ventral side of the body wall
  • Most broods have a fairly even ratio of male to female fish
  • Other species that are commonly mistaken for mosquitofish are the sailfin molly, the guppy, and the platty
  • As mosquito fish continue to take over more and more foreign bodies of water, scientists are now looking at ways to try to control their populations or remove them from the foreign ecosystems entirely. Their research has shown that human activities such as creating dams, which can slow down the flow of water, are actually helping to create even more suitable environments for mosquito fish and increasing their population
  • The eastern mosquito fish was introduced to the northeast coast of Australia in 1925. By 1934, it had spread to New South Wales, Southern Australia, and parts of Western Australia. Today it can be found in every state and territory in Australia with the exception of the Northern Territory
  • With the exception of Antarctica, mosquito fish are now found on every continent on Earth
  • Mosquito fish can travel through water as shallow as 3 mm. This allows them to be able to move between very shallow bodies of water as they dry up, or as the food source runs low. This is another reason they have been able to spread out so quickly across new ecosystems
  • Mosquito fish have been documented swimming in ice-covered waters in Japan, and also in hot bores in Australia that were over 99° F
  • Because of their ability to withstand salinity changes, mosquito fish have been able to populate not only fresh bodies of water but also salt lakes, estuaries, and near coastal marine environments. They have actually shown to be able to withstand salinity twice that of the ocean in testing
  • The juvenile stage for mosquito fish can last anywhere from 18 days to 8 weeks. This is mostly affected by the temperature of the water. Warmer water causes the fry to mature and grow faster, while colder water does the opposite
Mosquito Fish (Gambusia affinis)
Mosquito Fish (Gambusia affinis). Female Mosquito Fish (Top) and Male Mosquito Fish (Bottom).

How to Care for Mosquito Fish

Mosquito fish are incredibly hardy fish, able to withstand extremes in both temperature, salinity, and pH. They will also thrive in low oxygen conditions with poor water quality, which makes them a very hard fish to kill, even for a novice fish keeper. For this reason, they make a great addition to either an outdoor pond or an indoor aquarium.

Caring for Mosquito Fish in a Pond

Adding mosquito fish to your pond is a great way to control not just the mosquito population, but also other larvae too. They eat algae as well, which can also be beneficial. Because of their hardiness, they are a great choice for a pond because they can survive fairly extreme temperatures and habitat changes.

However, there are some downsides to consider as well. Mosquito fish breed rapidly, producing roughly 10-100 offspring every 24 days. If left unchecked, they can quickly overrun the other fish in your pond. They will also eat the larvae of fish and other bugs too, so they can be detrimental to the populations of other species.

All of this information needs to be considered before introducing mosquito fish into your pond. If it is a well-established pond with a large selection of species, it may be best to avoid mosquito fish. However, if you have ponds on your property that aren’t really cared for, adding mosquito fish to them can be a great way of controlling the bug population and algae. Some local governments even give them away for free as a way to control local mosquito populations.

Caring Mosquito Fish in an Aquarium

Due to how hardy mosquito fish are, they can make great fish for novice fish keepers. Their ability to survive extreme temperatures and water quality changes make them desirable for anyone looking to set up a new tank that won’t require too much attention. As they have been labeled as an invasive species in some countries, it may not be possible to find mosquito fish in stores in your location.

One thing that is very important to remember when adding mosquito fish to an already established tank is that they are known as fin nippers and can be quite aggressive at times, despite their small size. If you have any fish with long, decorative fins, or slow-moving, docile fish, you may want to reconsider adding mosquito fish into their environment.

Filter-type is another thing to consider when contemplating adding mosquito fish to an aquarium. As they are live-bearing fish, over the back filters with open intake tubes will suck most of the young fry up through it. It is recommended that a sponge filter is used, or at least a cover over the intake tube on over the back filters. If it is possible to remove the fry as they hatch then filter type isn’t too important.

Tank Setup

When setting up a new tank for mosquito fish, sand or gravel bottom are both suitable choices. However, gravel gives many more small hiding spots for fry, which will give them the best chance to survive and grow if you are hoping to successfully breed them.

Lots of cover and hiding places are required for fry to hide once they hatch, otherwise, the larger mosquito fish will eat them all before they can mature. Both real and artificial plants are okay in the tank, just keep in mind that from time to time mosquito fish will uproot and eat plants. Plants like hornwort and anacharis both provide lots of cover for fry and can also help your adult fish feel less shy. If you aren’t concerned with the fry surviving, a minimalist tank is also fine for mosquito fish.

 When setting up a tank for mosquito fish, it is very important to keep in mind that they can be aggressive towards slower-moving fish. They will often eat all the food in the tank before the slower fish can get to it as well. Because of this, it is often recommended that mosquito fish are kept in a tank with their own breed.

Tank Size

Mosquito fish can be kept in fairly small aquariums, with a single pair being more than comfortable in a 10-gallon tank. As they breed fairly fast, it is recommended that mosquito fish be provided with the biggest tank available. A single breeding pair of mosquito fish can over-populate a small tank fairly quickly.

Each mosquito fish should have at least a couple of gallons of water. The general rule of 1 gallon per inch can also be applied to mosquito fish.

Water Temperature

Mosquito fish thrive in water temperatures from 60° F up to 80° F. However, they can survive in temperatures ranging from 0° F all the way up to 100° F, which is one of the reasons they are such hardy fish. This is another reason they are a good choice for outdoor ponds, as they can withstand sudden changes in temperature if the weather changes significantly.

Water pH

pH levels for mosquito fish can also vary quite a bit, all the way from 6.0 – 8.0, giving them the ability to go into most established aquariums.

Mosquito Fish Size

When compared to other North American freshwater fish, mosquito fish are fairly small, reaching a maximum size of 2.8 inches for females, and 1.6 inches for males. The males are generally thinner than the females as well. When the fry are born, they are roughly ¼ – ⅜ of in an inch length. If being added to ponds or aquariums with bigger fish, some extra cover needs to be provided so that they can safely hide from larger fish.

Mosquito Fish (Gambusia affinis)
Mosquito Fish (Gambusia affinis)

Food & Diet

The diet of mosquito fish varies depending on whether they are living in the wild, in a pond, or in an aquarium:

What do Mosquito Fish in the Wild Eat?

In their natural habitat, mosquito fish’s diet consists primarily of larvae, small insects, small fish, small crustaceans, algae, and small amounts of plant matter. A single female mosquito fish can eat over 150 mosquito larvae in a single day. Despite this, and their name, they do eat larvae from all insects. They eat a wide range of insects as well, from beetles and mites to caddisflies and mayflies. They have also been known to eat their own fry if other food sources aren’t present.

What to Feed Mosquito Fish in a Pond

In a pond, mosquito fish will consume larvae, small insects, small fish, small crustaceans, algae, and plant matter. In a pond, they will also consume the fry of any other fish that may be in the pond, as well as their own fry if they aren’t well-fed enough. They are also known to eat eggs, both fish and amphibians, which can have serious impacts on the ecosystem they have been introduced into. Since they also eat algae, they can help to keep your pond algae-free.

What to Feed Mosquito Fish in a Tank

When living in an aquarium, mosquito fish will eat almost anything that is introduced into their habitat. They can survive on a diet of dry flake and pellet food, although it is recommended that their diet be supplemented with a natural food source, if possible. While it is possible to catch mosquito larvae or purchase them in certain places, they don’t necessarily need them in their diet in captivity. Foods like bloodworms, brine shrimp, daphnia, and scuds are the most common choices in place of larvae. In a tank, mosquito fish will likely eat their fry, which is why it is recommended they are separated at birth if you want the fry to survive.

When they were first introduced to new bodies of water to combat mosquito populations, it was believed that most, if not all of the mosquito fish’s diet came from mosquito larvae. This is now known not to be true and in fact quite the opposite. Research has shown that they can suffer high mortality rates when only fed mosquito larvae, and the fish that did survive showed poor growth and maturation.

Mosquito Fish Lifespan

The lifespan of mosquito fish can vary depending on their environment. In the wild, the average lifespan of a mosquito fish is 6 months, to 1.5 years. In captivity, they have been known to live considerably longer, up to 3 years and longer. Females have a longer life span than males do.

Mosquito Fish Breeding

Mosquito fish can be a very easy fish to breed, and for this reason, they make a great choice when selecting a fish to breed for the first time. The reason they can be a bit difficult to breed sometimes is the female can delay delivering the fry if she feels threatened. As the males can be quite aggressive, especially during breeding, a pregnant female will likely feel threatened in the main tank. It is best to move any pregnant fish to a separate tank until she has delivered her brood. At that point, she can be reintroduced to the main tank, and the fry can stay in the breeding tank. Having a ratio of 3 females to 1 male can also help to alleviate this problem slightly.

The male mosquito fish uses its gonopodium (modified anal fin) to insert its milt inside the female, which fertilizes the eggs. As mosquito fish are viviparous, the female develops the embryos inside her body, allowing her to give birth to live offspring. The gestation period is 26-32 days, although if the female feels threatened she will hold onto the fry for longer. Once she is comfortable, she will release anywhere from 10-100 fry, roughly ¼ – ⅜ inch long.

The most important thing when breeding mosquito fish is to separate the fry from adult fish. Even with lots of places to hide, most if not all of the fry will get eaten by adult mosquito fish before they get a chance to mature. Other than that, they are fairly easy to raise. Mosquito fish fry will grow rather quickly, approximately 0.008” (2 mm) per day. This growth can be slightly affected by water temperature though; the warmer the water, the faster mosquito fish grow. Maturation happens quickly as well, in roughly 3 to 4 weeks. This can be delayed slightly if they were born towards the end of the breeding season.

Fry can be fed finely crushed flake food, baby brine shrimp, micro worms, as well as many commercially available fry foods. If flake food is all that is available that is an acceptable diet until they are big enough to eat what the larger fish are being fed.

Female fry will reach sexual maturity in approximately 21-28 days, while it will take the male fry approximately 43-62 days. This is dependent on certain variables such as temperature and resources.

How to Identify Male or Female Mosquito Fish

There are several different ways to differentiate between male and female mosquito fish, including length, width, and fins. Females mosquito fish are larger, reaching a maximum size of just under 3” for females, and just over 1.5” for males. The males are also skinnier when compared to females.

The easiest way to tell the two sexes apart is their fins. The anal fins on adult females resemble the shape of their dorsal fins, while the anal fins of males are pointed. This is referred to as the gonopodium and is what is used to fertilize the female’s eggs.

The female can also be distinguished by a gravid spot at the rear of the abdomen, which grows larger whenever the fish is pregnant, and then fades again after she’s given birth.

Tank Mates for Mosquito Fish

Mosquito fish are known to be fairly aggressive fish, despite their small size. They will nip at other fish, damaging the fins of slower fish, and fish with long fins. For this reason, you do need to be quite selective when picking other fish to have in the same pond or tank with your mosquito fish.

Some fish that are okay to put in the same tank or pond are koi, goldfish, and carp. The important thing to remember is that all of these will grow considerably larger than the mosquito fish, so some of the mosquito fish may end up getting eaten.

Fish that shouldn’t be kept with mosquito fish are species like guppies, gouramis, and mollies. Anything that swims very slowly, or has larger than average, ornate fins, is not a good tankmate for mosquito fish.

Where to Find Mosquito Fish for Sale

Most mosquito fish are purchased from pet stores, however, there are still some areas in the United States that continue to give out mosquito fish for free to be used in private ponds for mosquito control. While these fish are okay to put in an outdoor pond, when adding fish to an indoor aquarium it is best to get them from a reliable source such as a pet store or reputable fish supplier.

Mosquito fish are very affordable fish, whether for a tank or pond. They can easily be found for around $2.00/US per fish, although they can be purchased for much cheaper if buying them in a large quantity. In some places, they can be hard to find as they are listed as an invasive species.

Are Mosquito Fish an Invasive Species?

Not only are mosquito fish listed as an invasive species, but they are also considered one of the most invasive species on the entire planet. Due to their use in mosquito control, they have been released into ponds, rivers, and lakes all over the world. Despite this, some argue that they are not any better at controlling mosquito populations as the native species are.

Once they’ve been released into a body of water, they multiply quickly. They eat not just mosquito larvae, but also other larvae that may have been beneficial to the ecosystem. They also eat fish eggs, frog eggs, small insects, small fish, and anything else they can find. Even though they get introduced to lower the mosquito population, they often lower the populations of most of the other species in the body of water too, including newts, frogs, and salamanders.

One of the main reasons they were used in biological control was to help control mosquito-borne viruses such as malaria. The reasoning was that if the larvae couldn’t hatch into mosquitos, then they couldn’t spread malaria and other diseases. Some people still attribute their use to the eradication of malaria in places such as South America, Russia, and Ukraine. There is even a statue of a mosquito fish in Sochi, Russia, to commemorate its help eradicating malaria there.

There use in biological control is still debated today. In some places they are listed as an invasive species, having done serious damage to natural ecosystems. It is now believed that it may have made the mosquito populations much worse when introduced to new ecosystems. Some scientists now believe that native species are better at controlling mosquito populations. But now, not only are the mosquito fish taking the native species’ food sources, but they are also eating their young as well, making the problem even worse. Yet in other places around the world, they are still being introduced into new bodies of water. India has only just started releasing mosquito fish into over 600 ponds within the last 10 years.

In the United States, in places where mosquito fish aren’t native, some states still have counties that give mosquito fish away for free for residents to put into ponds on their property. It is now believed that this has been one of the reasons behind the decline in small frog and newt populations in California. Despite this, California, and other states as well, still have counties that encourage and supply mosquito fish to control mosquito populations.

Due to constantly being released into new ecosystems around the world, and their ability to not only survive harsh conditions but thrive and multiply, they are now widely considered to be the most widely spread freshwater fish in the world. Once they are introduced into a new ecosystem, they establish themselves very quickly. Scientists have yet to find a way to a reasonable, repeatable way to remove them from an ecosystem once they are established. Some success has been had with draining small ponds, but this has an adverse effect on all the wildlife and isn’t repeatable on a larger scale.

Mosquito Fish vs Guppies

While they do come both belong to the Poecilia genus (which also contains swordtails, platys, and mollies), mosquito fish and guppies don’t really have very much in common. Besides breeding the same way (live births), being a similar size, and being from the Americas (Mosquito fish from North America, and Guppies from South America), they don’t really share many other similar traits. 

While mosquito fish are fairly bland, guppies are extremely colorful and flashy and have endless color possibilities. Guppies also have large tails in comparison with their bodies, while mosquito fish have much smaller tail fins. Mosquito fish can be aggressive and prefer to stay with their own species, while guppies are very social, and are quite happy to be with other species of fish.

One thing that is shared is their use in biological control. Guppies are also very good at eating huge quantities of mosquito larvae. With the spread of the Zika virus over the last few years, guppies have been released in stagnant, sitting water in South and Central America, Africa, the Caribbean, as well as areas in South Asia. Even though India has already been using mosquito fish to control mosquitos, they have also been releasing guppies as well. They are currently being used there to combat both the Zika virus and Dengue, with fairly good results so far.

Western Mosquito Fish vs Eastern Mosquito Fish

Western mosquito fish (Gambusia affinis) and eastern mosquito fish (Gambusia holbrooki) are fairly similar in many ways, including diet, appearance, origin, and giving birth to live fry. Both are part of the same genus, and both have been introduced into foreign waters all over the world to eat mosquito larvae.

While western mosquito fish originated from the Mississippi River and its tributaries, as well as the streams and rivers of Indiana and Illinois, the eastern mosquito fish originated in just the southeastern United States. They are both most likely found in shallow, standing to slow-flowing water. Both species are incredibly tough when it comes to water temperature, pH levels, water acidity, and even chemical changes to the water. Even human spread and environmental loss and pollution haven’t seemed to have any effect at all on either species’ population.

Eastern mosquito fish are considered a noxious pest in Australia, and they have caused extensive damage to many native species. Not only have they damaged fragile ecosystems, but they have also not even helped, and possibly made the mosquito problem worse. Australian scientists now believe that the native species were better at controlling the mosquito population than mosquito fish are. When they were originally introduced, it was believed that their diet would consist of mostly, if not all, mosquito larvae. However, this turned out not to be the case, and they also started eating other larvae, fish, as well as frog eggs, and tadpoles.

They have now been implicated in the decline of at least 9 fish species, and 10 native frog species. Because of their high reproduction rate (an average of 50 fry per brood, with an average of 9 broods per year), quick maturity, and aggressive nature, they can out-compete almost every other species of fish native to Australia. In some parts of Australia, it is against the law to transport mosquito fish from one location to another, to help try to mitigate the spread as much as possible. 

Classification of the two species has been difficult due to their similarities. However, it is now believed that the eastern mosquito fish could actually be considered a subspecies of Gambusia affinis.