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Killifish
Note: Many thanks are also due to Doug Dame for his extensive additions to the section on American killifish.
There are very many killifishes suitable for the brackish water aquarium, although only a few are regularly traded in commercially. This is a shame, as killifish are small and colourful, and hardy enough to do well in most domestic aquaria. They are relatively straightforward to breed, although may need quite specific water conditions to spawn. Most species are distinctly territorial, and while they don’t require huge amounts of space each fish should be allowed enough space to set up a home patch without interfering with the other fish in the tank.
Killifishes are most diverse in the tropical part Americas and Africa, especially in shallow or seasonal waters where other fishes are lacking. There are also killifishes in Asia, and the temperate parts of North America, Mediterranean Europe and the Middle East. Identifying species is not easy, and only a superficial review is made here. Many books have been published on keeping killifish in aquaria, but these tend to concentrate on the African and Asian species. For the European and American species you may need to refer to books on native fish faunas. There are also a number of killifish associations, some of whom have web pages. There are many advantages to belong to these associations beyond access to information, including the opportunity to get specimens of killifish not otherwise traded in commercially. One great feature of keeping killifish is that many species lay eggs which, if wrapped in damp moss or cotton wool, can be mailed around the world! In many cases it is also possible to catch specimens of native killifish yourself. As with all native fishes, check with your local authorities as to the legal restrictions on collecting in your area, and always collect responsibly. Some species are rare and protected in the wild, and these should never be collected, and with those species that are not protected, take no more than you really need and do your best not to otherwise disrupt the habitat from which you are collecting.
American Species
The most widely traded brackish water killifish is the Florida flagfish Jordanella floridae. This is a medium sized killifish, growing to about 6 cm (2.5 inches) in length, and rather deep and stocky. The fish are very attractive with a complex pattern of red and pearl coloured stripes along the body overlaid with green or blue bands most obvious on the upper part of the flanks from the head back to the dorsal fin. The dorsal fin is also brightly patterned. Although both sexes share the same basic colouration, the males are especially colourful when in breeding condition. These fish occur in a wide variety of waters, not just brackish, so some care should be taken to allow the fish to adapt to the conditions of the aquarium. Otherwise they are active and hardy. As well as flake and small invertebrates, these fish should be provided with algae and other vegetable matter. Like many killifish, nibbling at algal growths ("aufwuchs") is an important source of food in the wild. Keep these killifish in a planted tank, but not too densely stocked since the males can be territorial. This is a subtropical fish and can tolerate a wide range of temperatures, from 20 to 30° C (68 to 85° F). It will do better in warmer conditions though, and in some cases too low a temperature may be the factor causing some males never to develop their otherwise impressive colouration. A specific gravity of 1.002 to 1.005 is sufficient, and hardy varieties of Vallisneria and Sagittaria are similar enough to the natural plants of their native biotope. Since this is a fish naturally found in quiet waters, avoid using powerful electric filters and instead opt for something that produces a more gentle flow of water such as a small internal electric canister filter or air-powered undergravel filter. The closely related Yucatan pupfish Garmanella pulchra needs similar conditions, but is less tolerant of low temperatures and must be kept between 22-28° C (72-82° F).
The American killifish of the genus Cyprinodon, such as the sheepshead minnow Cyprinodon variegatus widespread along coastal North and Central America, are very characteristic of costal brackish and marine habitats and can often be found alongside livebearers such as mollies. Besides estuarine waters, Cyprinodon species can also be found in inland waters made saline by mineral-rich streams or high rates of evaporation, and also in hypersaline lagoons, where these fishes can be found in waters up to six times more salty than normal sea water. Unsurprisingly then, it is important to provide these fish with hard, alkaline water (pH 7.5 to 8) well buffered with coral sand or crushed shells placed in the filter. Most Cyprinodon require plenty of warmth and a rich growth of algae on which to graze: a temperature of above 25° C (77° F) is essential as is bright light to promote good algae growth but do provide some cover for the fish with floating plants or small caves (small flower pots are ideal). Avoid using materials, like driftwood, that might lower the pH. Although not as attractive as Jordanella, in breeding condition the males often becomes brighter and quite attractive. In the case of the sheepshead minnow, both sexes are a drab olive-brown most of the time, the male developing a bright metallic blue region across the upper part of the flanks when in breeding condition. Provided with the right aquarium the most attractive thing about these fish is their behaviour. Sheepshead minnows are active fish, and in the case of males territorial as well, and so do best in a species tank where they won’t be able to harrass other species. Males hold open water territories about 22 x 22 cm (9 x 9 inch) square, preferring sand or a mixture of sand and gravel over more complex bottom conditions filled with ornaments, bogwood or pebbles. They spend a great deal of time chasing one another out of their home patches. As such then, these fish would be ideally suited to a fairly spacious open species aquarium with floating plants rather than the more typical community tank, with some cover towards the edges for fishes to hide in but most importantly enough space for all the males to set up their own territories. There are numerous other Cyprinodon species that can do well in aquaria but these need to be collected from the wild as they rarely, if ever, turn up in aquarium stores.
Another American killifish found in a variety of waters are those of the genus Fundulus. These fish have been widely studied by zoologists because of their remarkable ability to adapt to different water conditions from freshwater right through to hypersaline water more salty than the sea. The mummichog Fundulus heteroclinus is perhaps the best known. These fish are substantially larger than Jordanella or Cyprinodon, reaching as much as 15 cm (6 inches) in some cases. In shape they are streamline and rather like minnows without the upturned mouths of many other killifish. They are quite predatory, taking small insects and crustaceans mainly but will eat small fish and eggs if they can. Otherwise they are peaceful and should be mixed with other quite fish. In the wild they are commonly found with fish such as mollies Poecilia and Mollienisia. These make ideal companions in captivity, since they are of similar size and temperament but prefer the upper part of the tank, while Fundulus tend to swim close to the bottom. Being subtropical fish these fish do not need quite so much heat as tropicals, around 20° C (70° F) being adequate, perhaps dropping by a few degrees in winter. Closely related species include two similarly large but rather less easily maintained species, the longnose killifish, Fundulus similis, and the Gulf killifish, Fundulus grandis. These species are naturally found in marine or brackish waters rather than freshwater and this may be one reason why they have a reputation for being difficult to maintain and are much less commonly seen than the mummichog.
There are some other North American killifish worth trying out but as with many of the species mentioned here these will need to be collected as they are rarely traded. The diamond killifish, Adinia xenica, is a small (5 cm / 2 inch) omnivorous species that likes a deep aquarium with marine, brackish or hard freshwater (try to provide conditions as close as possible to those that the fishes were caught in). It is very pretty, with bright pearly white bands and almost diaphanous fins with a beautiful lacy pattern to them. The rainwater killifish, Lucania parva, is another small species that can be found in a variety of water conditions in the wild. Both these killifish do best in a mature tank and may need to be offered live foods at least initially. The very attractive gold-spotted killifish Floridichthys carpio is a medium sized killifish, up to 7.5 cm (3 inches) found in brackish and marine waters that resembles the sheepshead minnow in shape and requirements, but as the name suggests has a very pretty pattern of golden spots on the body and fins.
European & Asian Species
The European and Middle Eastern genus Aphanius is rarely traded but in many ways parallels the North American Cyprinodon in habits and maintenance. Some species of Aphanius are extremely colourful. An example is the lovely Aphanius mento in which the males are blue-black in colour with brilliant light blue spots. These fish should be snapped up when seen! In most cases the males are territorial and the fish should be kept in a large tank, around 80 cm (36 inches) long.
The Asian killifish of the genus Aplocheilus such as the Ceylon panchax Aplocheilus dayi and the striped panchax Aplocheilus lineatus are among the commonest killifish traded. They are known as "panchax" after the genus Panchax to which many of these species were once assigned. They are found in a variety of waters and will adapt well to slightly brackish water with a specific gravity of up to 1.005. These are fairly large killifish, up to 10 cm (4 inches) in length. They are distinctly predatory, and can easily eat fish of male guppy size. Kept in a well planted tank they mix well with fish too big to eat, such as glassfish (Parambassis spp.), chromides (Etroplus spp.), and small spiny eels (Mastacembelus spp.) which all live in the same habitat.