Mine used to harass each other and other fish. In a pond setting, they are less bullying. They also will eat some of their babies. Each fish should have a few gallons to itself but can thrive in high densities with more than five fish per gallon. They will live at least a few years.
Mosquito fish are native or introduced to much of the United States.
Here in Maryland, they inhabit many drainage ponds and ditches. They are used for mosquito control in stagnant water. More often mosquito fish will eat other tasty insect larvae and fish fry. One scientist examined the stomachs of over 2000 mosquito fish and found only a small percentage of mosquito larvae. Instead, aquatic insects and fry made up most of their diet.