There are over 1000 different species of killfish belonging to several different families. The vast majority of killfish are small and range up to approximately 2 inches in length (5 cm). As there are so many different types of killfish, the group has a wide variety, with species found in Africa, southern Europe, and America. All species are oviparous, which means that the mother releases undeveloped eggs. Most species have interesting colors and are also easy to keep in aquariums, making them popular as pets.
The largest family is Rivulidae, which contains over 300 species. There are several other families such as Valenciidae, Aplocheilidae, and Cyprinodontidae. In some areas, the term killfish is used to describe any fish that are part of the Cyprinodontidae family, while in other regions it has a much broader meaning.
In general, killfish are small and grow to a few centimeters. There are, however, some species that do grow. The largest species grows to about 15 cm in size. This is another reason why fish are regularly kept in aquariums; they require less space when compared to many other types of sea creatures. Typically, the fish lives for about two years.
The variety of this type of fish is great, as different species are found in the waters of most continents. Antarctica and Australia are the only two continents where no species can be found, but northern Europe has no killfish either. Most species live in lakes and rivers, but there are others that have adapted to more temporary water areas such as floodplains.
All killfish are oviparous, which means they lay eggs. To be classified as an oviparous fish, the mother must release eggs before any significant development takes place within her. Most, but not all, fish stay young this way.
Killfish fish are known for being easy to keep as pets. Fish can adapt, and even reproduce, in an artificial environment with few problems. A potential problem with some species is that they can become aggressive towards other species, so it is important to mix fish carefully. Most species are brightly colored, which makes them especially attractive to fish enthusiasts. A particularly common species is the Golden Wonderor, also known as the Striped Panchax, which can often be found in pet stores as well as merchants specializing in aquarium fish.