Killifish as a group comprise over 700 species from most regions of the world. Some are easy to keep in the aquarium, others are difficult. Most are brightly coloured, others, have fascinating behaviour. This site contains the basics on how to keep killies, fish photographs and breeders tips and my own killiblog. This site currently contains 435 pictures of 188 different species. Check out the most recently added killifish images.
Generalisations on keeping killies are somewhat few and far between, as these fish inhabit such a wide range of environmental niches worldwide, and have therefore adapted to very wide range of conditions. Many people have an idea that these fish are difficult to keep, and whilst there are some easy killifish species, there are many that can test the skills of the most experienced fishkeeper. Actually most killifish are quite easy to keep, especially in a species tank (i.e. not a community tank), and the 'difficulties' are more related to breeding them.
However the following generalisations are largely true
So, to succeed at keeping killifish, and to answer the generalisations above, you'll need to: 1) continually breed your fish, 2) continually breed (or collect/buy) livefoods, 3) keep one (or more) tank / species. All of this is time consuming and takes up space. Successful killifish care is not hard, but requires a significant commitment in time. However, unlike many hobbies, there is very little expense required beyond buying starter cultures of livefoods and the eggs/fish themselves.
The majority of killifish maintained in captivity hail from Southern and Central America, Africa and Asia with limited numbers from Southern Europe, the Middle East and North America. By far the most popular, largely due to their intense colouration, are the annual fish of East Africa and South America, and the non annual fish of West Africa and Central America.
Annual killifish are those that live (usually) in temporary waters: pools that dry up on a regular basis. As such, these fish have to cram their life cycle into as a short a period as possible, hatching, growing and reproducing in a few months only. The best examples are the Nothobranchius species of south and east Africa, and the (formerly) Cynolebias species of south America. The eggs of these fish have adapted to survive in the mud of a dried up pool, and are ready to hatch when the wet season starts.
Species that live in permanent waters are far more akin to other fish species, and are perhaps less of a challenge to the new killikeeper. The best known non-annual killis are the Aphyosemion and Fundulopanchax of west Africa, and the Rivulus species of south and central America.
Due to the fact that many killifish species inhabit a very small geographic area, they are particularly susceptible to extinction by habitat destruction. Numerous species are thought extinct, for example Nothobranchius steinforti has not been found since 1976, despite extensive searches. Others, such as Nothobranchius capriviensis are likely to be lost by construction projects that remodel the landscape in which their temporary pools would occur. It is the aim of many killifish keepers and societies to maintain captive populations of as many species as possible, to safeguard against future extinctions. For this reason, it is imperative that fish with known collection data are not cross bred with other similar-looking fish, as this may result in sterility in captive populations.