Molly (Poecilia sphenops)
Mollies originate in the fresh water streams and coastal brackish and marine waters of Mexico, where they are a dull silvery colour, often sprinkled black all over. Commercially bred mollies come in a wide variety of colours, however. They are live-bearing fish that are fairly easy to breed, and in fact fairly difficult to keep from breeding if you have them in a tank with both males and females, though if left in a tank with other adult fish, including their own parents, the baby mollies will get eaten. If you keep males and females together, firstly make sure you have a decent ratio of female to male (at least 3:1) to prevent the females being too pestered by the males; also watch out for pregnant females and be prepared to rescue newborn fry to keep elsewhere (in a designated fry tank, with the same water parameters as the home tank) until they are big enough to fend for themselves.
Originating from coastal waters, mollies can live in slightly salted or brackish water, but this is by no means essential to their well being; in fact, it is now thought to be something of a myth that even wild mollies spend a lot of their lives in the most coastal areas, instead nipping there for breeding and then making their way back to fresher water for the rest of the time. In any case, mollies bred and bought commercially will almost invariably have been brought up in freshwater aquariums, so will know and desire no different. In a tropical set-up, they can live alongside other live-bearers and peaceful community fish, but are best avoided around fish with long or decorative fins, as they will be likely to fin nip. Mollies will eat a range of flake, frozen, freeze dried and live foods.
Also available are the Sailfin Molly (Poecilia latipinna), Lyretail Molly (a common molly with an unusually shaped tail, selectively bred) and Balloon Molly (which is basically just another variety of common molly, selectively bred to be a different shape, originating from a genetic defect.)
Size (fully grown): 12cm (4.5″) for females; 8cm (3.2″) for males
Water temperature: 25 – 28C
Water pH: 7 – 8