Among freshwater fish none have more beauty and grace than the killifish. Their range of prismatic colors, their gossamer fins and their slow elegant movements, make these small minnows the focus of any aquarium. Perhaps best of all is that they are not difficult to keep, need little special attention, do well at a wider range of temperatures than most tropicals and can be easy to breed.
Many hundreds of species live in the wild all along the tropics, except in China and Australia. They live in a range of circumstances from slow moving streams to small seasonal pools of water. This latter group lays their eggs in those pools as part of an unusual reproductive cycle that is highly appreciated by killifish enthusiasts.
The pool evaporates and the eggs lie dormant until the water returns and they will hatch. The enthusiasts (and there are many worldwide with perhaps more clubs and associations devoted to killies than to any other fish) will often send dormant eggs to each other through the mail.
No descriptions of these fish can do them justice. Some are rainbow colored with a wash of red and blue over silvery bodies that become darker and lighter depending upon their breeding conditions. Even those fish colored in a muddier palette still bear a jewel-like tiger's eye shimmer.
The favorite aquarium species are from the genera Aphyosemion, Epiplatys, and Rivulus. Common names for these fish are often very localized and breeders and hobbyists have learned to cope with the Latin designations. These fish are only rarely carried in pet shops since they come almost entirely from breeders and can be expensive relative to the usual aquarium fishes. They also do not do well in large groups so it's difficult for aquarium shops to keep a large quantity on hand.
The smallest of the killifish are no bigger than half an inch and the largest no bigger than 6 inches. They do well in pairs, or as a male with two or three females, in a small tank. Five gallons is fine for a pair. A heater is not necessary if the temperature of the room in which they're kept remains steadily between 68 and 75 degrees Fahrenheit. As long as the temperature doesn't drop too drastically, these fish can even deal with slightly lower temperatures.
Since they inhabit small springs, streams, and pools in the wild, a great deal of water motion is not recommended. They seem to like it calm and so a small box filter, while nearly obsolete for community tanks, can perfectly serve a 5-gallon killie tank. The trick is to maintain the filter well. If there is one thing that killies are sensitive to it's good water quality.
Use a sponge filter to keep a good store of nitrate-eating bacteria or wash out the carbon or gravel with aquarium water rather than fresh water at least once a week. Change a quarter of the water twice a week and replace it with de-chlorinated water of the same temperature, preferably water that has aged a couple of days in an open tank.
Aside from when they're mating, killifish become lively specimens when they're feeding. While they may take dried food such as tubifex worms, they seem happiest with live food. Bloodworms are the perfect food, but other live food from insect larvae to tiny mealworms to brine shrimp will serve. They will quickly come up and feed at the surface and even take food right from your fingers. Be careful when you remove the top, however, because killifish are terrific jumpers and can easily leap out of the tank. Best advice is to keep a screen top over the tank beneath the hood.
Killifish don't need a great deal of light. If you have them in a tank with live plants that need a lot of light, provide a darker area of the tank for them. They do perfectly well in a tank without plants, and their colors seem to brighten against darker surroundings.
To obtain killifish, ask your local aquarium shop if they know of local breeders or find a breeder through one of the many killifish associations on the Web.