Heterandria formosa female

Scientific Name: Heterandria formosa

Common Names: This fish is most commonly referred to as the Least Killifish. This is actually a misnomer since it is not a killi but is in fact a livebearer. It is also called the Mosquito Fish not because of it's diet but because it's small, grey, striped body resembles that of a mosquito.

Appearance: The most striking thing about these fish is their size. Females grow to just over an inch in length while males rarely exceed 3/4 of an inch. This ranks them as the smallest live bearing fish! The fish are grey with black vertical bands along the length of their body. Their bellies tend to be lighter than the upper halves of their bodies, and this is especially apparent on the females.

Native Habitat: These fish are native to the costal plains of the southeastern United States. They exist from the Cape Fear River drainage in North Carolina to southern Louisiana. They are abundant in peninsular Florida. There are isolated introductions elsewhere as well. The fish are found in heavily vegetated standing or slow moving fresh or brackish water.

Water Conditions: These fish are very tollerant of a wide range of water conditions. They thrive with a pH between 6 and 8 and a hardness of 15-20 degrees. The temperature should be in the mid 70s (Fahrenheit) but some fluctuation is accepted. In water that is kept too warm they will grow too fast and be weaker as adults. In cold water they can become stunted and fry survival rates will decrease. They will adjust to most tank conditions, however. They are also well suited for garden pools.

Poor water conditions such as those found in a cycling fish tank are tollerated well by these fish. They will continue to reproduce albeit with lower survival rates even in a tank with no circulation or filtration. This is not recommended, of course, but speaks well of their toughness.

Tank Setup: Because of their small size, these fish can thrive in tanks as small as five gallons. They will survive in tanks much smaller than this but will not be as interesting to watch.

A tank housing these fish should be heavily planted with fine leaved plants such as milfoils (such as M. pinnatum, M. sibericum, etc) and Java moss. Check your local regulations about which milfoils you can keep as many are listed as invasive.

A simple air driven sponge filter will do well with these fish. In many cases you can keep them without a heater since they are not tropical (see Water Conditions). Any kind of filter with an intake should have it covered so that these small fish do not get sucked into the impeller.

Feeding: These fish will accept just about any kind of food you give them. Flakes will have to be crushed to make them easier to eat with their small mouths. Their diet should be mostly vegetable, but some protein such as earthworm or brineshrimp flakes can be included as well.

Breeding: These fish are very easy to breed. If you have males and females together, the females will get pregnant. They do not give birth the same way as other livebearers do. Their small size limits how many fry can grow inside the females and because of this they have developed a reproductive system that resembles an assembly line with one or two fry being born every couple of days. The adults do not eat the fry and you will always have fish of all ages to observe.

Aggression: These fish are harmless unless you are a piece of crushed flake food or a daphnia Smile

Comments: My fish have thrived in every condition that can be found in a ten gallon tank. I once took the plants out of their home to catch them and give them away and was stunned to find over 200 fish living in there! They showed no signs of stress even with this overcrowding however, I don't suggest you do this on purpose. When I got home from the pet store that I took them to I realized that the tank that I had put the plants in had a few fish that must have been caught in them. Within a month it was full again and I was back where I started.

Resources:
Page, Lawrence M., and Brooks M. Burr. Freshwater Fishes. Boston, New York: Houghton Mifflin, 1991.

© 2004 The Fishie Zoo