A splitfin is a fish in the family Goodeidae, part of the order Cyprinodontiformes. There are around 40 species in this family and some are threatened or endangered, while others have a relatively stable conservation status. Several species are kept as aquarium fishes including the Redtail Splitfin and the Butterfly Splitfin. Aquarium enthusiasts can order splinfins through catalogs or stores that stock fish for hobbyists. Some people who keep these fish are also willing to trade or sell as long as their fish are breeding and producing young.
This family of fish is named for the distinctive split that can be observed in the anal fin. Part of the fin is almost completely separated from the other part and this physical feature is highly visible and easy to identify. Most splitfins are relatively small fish, although some can grow as long as eight inches (20 centimeters). Along with other teleost or ray finned fishes, the splitfin has a distinctive protruding jaw and a symmetrical tail fin. Their coloration varies depending on the species and some fish have markings like stripes and spots that assist the fish with camouflaging in their natural environment.
Splitfins prefer warm fresh to brackish waters. They are found in Mexico and the United States with some species like the Railroad Valley Splitfin living in more extreme environments like warm springs. Feeding habits vary but many are omnivorous, consuming plants, algae, and smaller fish species. Their coloration helps them evade larger predators while also providing concealment for hunting prey fish.
Many splinfin species are viviparous. The female fish are fertilized internally and the eggs hatch inside the female's body, allowing her to bear live young. These fish have not been widely studied because they are not valuable as food fish and thus are of no interest to fisheries researchers, and while they are kept in aquariums, they are not a hugely popular aquarium fish. The lack of information about many species is a barrier to conservation efforts.
Conservation concerns about some splitfin species mainly surround fish with very limited habitats. These fish are vulnerable to human activities that alter their habitats and make it difficult to survive as they may not be able to thrive if relocated. Fish populations in limited river systems or inland bodies of water like lakes can easily be rapidly depleted by fishing, introduction of other fish species, pollution, changing water temperatures, and radical changes to water level.