Mosquito fish and guppies are both members of the poecilia genus, a designation they share with mollies, swordtails and platyfish. They are both small freshwater fish that give birth to live young and are not above cannibalizing them. The anal fins of the males are modified into reproductive organs, and both guppy and mosquito fish females have a "pregnancy spot" on their bellies during the reproductive period. But they do have some differences.
Mosquito fish grow to 1 1/2 inches long and are robust but compressed. They're tan to olive above and pale yellow below, and the scales have darkish spots near the edges. The male may have a blue shimmer on the sides. The female is larger. There's also a dark bar beneath the eye and spots on the dorsal (along the midline of the back) and caudal (on the hindmost part of the body) fins. Guppy males grow to 1 1/4 inches long, while the females grow to 2 1/4 inches long. They have large eyes, and their color can be varied and spectacular, especially in domesticated guppies bred to be colorful. Wild guppies sport irregular black spots or patches, while the sides have shimmering colors of red, blue and green.
Guppies are native to northern South America, Barbados, Guyana and Trinidad, where they like still or flowing waters. In the wild, the mosquito fish lives near the surface of fresh or brackish water in ponds, lakes, ditches, backwaters and slow streams from New Jersey to central Mexico. They can also be found in the Mississippi River basin from Illinois.
Needs in Captivity
The pH of the water isn't important for mosquito fish, and the temperature should be room temperature. In the wild they can stand temperatures just above the freezing point of water (37 F) to 86 F. The mosquito fish is named because it eats and prefers mosquito larvae and has been introduced into areas to control the mosquito population. They are good eaters and can consume their body weight in food every day. The guppy needs lots of varied food, and prefers hard or very hard water, which means water with lots of minerals dissolved in it. They like water between 68 and 75 F with a pH of 7 to 8.
Mosquito fish are somewhat irascible and don't like to live with other fish. As aquarium fish, they're best kept in tanks with their own species. The guppy lives in loose schools where it's always moving. The guppy, unlike the mosquito fish, is easily kept with other live bearers.