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Heterandria formosa

Reverse trio of Mosquitofish, Heterandria formosa


When I was asked about what was my favorite livebearer, I had to think hard.  There are many different species that I enjoy keeping for their interesting fins, colors, disposition, body shape, or behaviors.  My choice of a favorite may surprise some people.  Because of past pleasant experiences, I chose Heterandria Formosa, the Mosquitofish, Dwarf Livebearer, or Least Killiefish.  Its diminutive size makes it adaptable to even the smallest of tanks, and nano aquariums are very popular now.  Adult males only get about .8 inch and females only about 1.4 inches, and in fact, it is the smallest of all livebearers and the 7th smallest of all fish species!  It comes from the South Eastern US, including Florida and adjacent coastal states, inhabiting shallow, sluggish, weed infested fresh waters.  It also can be found in brackish waters.  PH is best between 7 and 8 and it likes some hardness in the water.  Because it is a US native species, it does not need a heated aquarium and 68 to 78 degrees is a good range, although it will survive well above and below that.  Needless to say, the mosquitofish is a hardy and durable species of livebearer that is easily kept. 


Feeding and breeding this fish is a snap also.  It will eat most all foods, despite its small size, and will tear apart and eat blackworms, frozen adult brine shrimp, daphnia, and of course, flake foods.  Some vegetable matter is important, so some spirulina flake is recommended.  It is a very peaceful fish, even among its own kind, but somewhat shy, so a good number of plants are helpful in the aquarium.  A species tank is probably best, although they will do well with other small peaceful fish, just so the tiny males do not get eaten!  It is a member of the Poeciliidae family of livebearers, but instead of having all her fry at once, the female will drop a fry or 2 every few days for about a month.  If there is plenty of plant cover and the parents are well fed, the fry can be left with the parents.  They will eat fine dry food and baby brine shrimp right away.


This fish is not colorful, but very attractive.  On a grey body, a prominent black horizontal line goes from the eye to the tail and there is a black spot on the dorsal fin.  The female also has a black spot on her anal fin.  This gives the Mosquitofish a prim and perky appearance, and their fins always seem to be fully extended, a sign of good health.  The male has a gonopodium that is 1/3 his body length.  The Mosquitofish is easy to keep, attractive, fun to watch, and has many interesting behaviors.  I could describe it in one word that I am normally reluctant to use: “cute”!!