killifish is derived from the Dutch word

The name killifish is derived from the Dutch word "kilde", meaning small creek, and puddle. Most killies are small fish, from one to two inches, with the largest species growing to just under six inches. They comprise of a group of fishes of the family Cyprinodontidae and are commonly known as the egg laying tooth carps, cousins to live-bearing fish such as mollies and guppies. Like their live-bearing counterparts, killifish are usually mosquito larvae eaters. There are about 350 species of killifish found within the equatorial belts throughout the world and in practically all regions except Australasia. Many killifish species are as brightly colored as salt-water fish and have incredible finnage.

In the wild most species are territorial by nature, the male defending his area against intrusion by other males. They mate with whichever females they can tempt into their territory. In the aquarium, it's best to keep a ratio of one male with two or three females. Killies are excellent jumpers so keep a closely fitting lid. In their natural environment most killifish live in soft and slightly acidic waters (pH 6.2-6.8) with temperature ranging from 22 to 26° C.

Killifish are divided into three general breeding groups of annuals, semi annuals, and non-annuals (plant spawners). In the wild, the annuals spawn every day, because in nature, when the pond dries up, the eggs go into a hibernation phase known as diapauses. When the pond fills with the rains from the next season (sometimes 6 months) the eggs hatch and it isn't long before the whole pond is filled with killifish. Spawning is done every day, just in case in the next rainy season, the first rains are insufficient to fill the pool. By spawning daily (2-3 eggs or more), this ensures hatching times of the next generation of killies are thus spread out over time. Species of annual killifish therefore grow and mature quickly, so they can repeat the process before the ponds dry up again, between rainy seasons. Killifish that come from areas that are wet year round, such as streams, ponds, and swamps, tend to livelonger, some as many as 5 years or more.

Semi-annuals live in areas, which sometimes dry out to moist mud, but at other times retain water throughout the dry season. They are therefore substrate (in the mud, leaves, etc.) spawners and tend to live a little longer than the annual species of killifish. Semi-annual killifish can be water incubated, but also have higher hatching rates if allowed to be stored in moist peat for 30 -60 days. After this 'drying' period put the peat and eggs in water and within a few days’ baby fish hatches. Semi-annuals generally tend to be substrate spawners, meaning they will spawn in peat moss, Java Moss or in spawning mops placed at the bottom of the aquarium.

The non-annuals, such as the Aphyosemion genus species of killifish, live in permanent bodies of water and, in some cases, will live for up to five years. These species, although there are some more difficult and require more to keep, breed, and raise species. The non-annuals, or plant spawners, can be bred using the spawning mop method for higher yield and with water incubation of the eggs. Most non-annual killifish eggs hatch on an average of 21 days or so.