The Mollie, All You Need to Know but were Never Told

Mollies, Americas Native Fish; History, Care and Breeding Explained


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A Short History

The Mollie or Mollienesia, as it was originally classified, has some of the most beautiful and unusual varieties of Live-bearer fish available today. For example; The Sailfin Mollie is arguably one of the most beautiful and exotic fish available of any species. However, Mollies are perhaps the most difficult of all of the common Live-bearers to keep. Under the normal recommended community tank conditions, the Mollies seem to quickly become sickly, listless and even die.                  

Members of the Mollie family are arguably the most delicate of live-bearers. When chilled or otherwise exposed to environmental stress of any kind, they tend to develop an ailment known commonly as “shimmies”. In reality, this is not really a disease but merely symptoms of stress, much as some salt water fish would display under similar circumstances. 


How to Spot and Cure “Shimmies”

A Mollie with “shimmies” rocks back and forth and seems to be shivering, it appears to be swimming but not getting anywhere, also its fins appear tightly clamped to its body. If nothing is done, the Mollie will die very shortly, however, remove the stress and the condition corrects itself remarkably fast. 

Two things need to be done, first raise the temperature to the high 78F, and second add one to two teaspoons per gallon of salt to the water. I mention this, because Mollies have discouraged many the new aquarists and yet is so common a problem, not your fault and easily avoided.


The Sailfin Mollie

The Sailfin Mollies are among the most dramatic of all livebearers. They are also the largest and most difficult to maintain. Mollies normally inhabit brackish (salt/fresh water estuaries) environments, and do best in captivity in hard, alkaline water with added salt.

In nature Mollies are nearly exclusively plant and algae eaters, so they must be fed lots of spirulina, even boiled spinach finely chopped, to remain healthy. Sail fin Mollies are available in several color varieties. Only males have the “sail fin” and in the wild, can grow up to 5 inches long. Although these fish breed readily, young raised in tanks rarely develop the “sail fin” and in fact only reach about 3-31/2 inches in length. But raised in a back yard pond, astounding, jeweled sail fins will appear at about 4 months.


Is the Mollie Related to the Guppy?

The genus Mollienesia, from which the name Mollie was derived, has been re-classified as Poecilia and considered to be in the same genus as the Guppy. For years it had been noticed that the guppy will, under certain circumstances, interbreed with the Mollie, and now the close relationship between the two has been confirmed.


A Mollie’s True Nature

The native habitat of these fish extends from the southern United States to the Yucatan in Mexico, and they thrive mainly in brackish estuaries. Their maximum length is about 5 inches, but in the aquarium they only reach about ¾ or less of its potential length. Mollies of all varieties like plenty of warmth, a temperature of at least 78F is recommended. 

Mollies are not very aggressive, but can protect themselves, so they are suitable tank mates for almost all tropical community tank fish. Mollies require a well lit tank with plenty of live plants (which they will nibble at) and some alga is always recommended, they like to graze new algae growth. 

They are not, in any way demanding when it comes to water quality, however the addition of sea salt at the rate of 1 teaspoon per gallon is recommended, and with most US based tap water required. The sail fin Mollie must be given at least a 20 gallon long tank to swim in or he will probably never show the beautiful sail fin, and will diminish in health and eventually die.

In short, all of the Mollie species have been interbred, resulting in many different color varieties and finnage shapes. We now have the solid black mollies, both sail fin type with orange-bordered dorsal fin and the sphenots, or common variety. There are other black aquarium fish but no freshwater fish approaches the velvety black texture of the Black Mollie. Albino forms with red eyes, there are true lyre tails in most every color, even chocolate, gold dust and orange varieties!

Mollies, like Guppies, can be acclimated to full strength sea water. They enjoy eating algae growth in the aquarium, and will graze endlessly on it, looking for choice patches to nibble. IF your tank lacks algae, you must provide “spirulina” flakes, special commercial Mollie food, or small amounts of chopped boiled spinach. Mollies are mostly found from Southern United States down into Central America.


Can Mollies from Florida Lakes Live in Home Aquariums?

The little white and black spotted fish so common in of the lakes Florida may look like Marble Mollies, but they are most probably not. It is never a good idea to mix wild fish with store bought farm raised and tank acclimated aquarium fish. The nature of wild fish is much more aggressive and they can introduce parasites and strange diseases from their lake or stream of origin.


Fun Fact about the Mollie

As we said earlier, Mollies live in both brackish and fresh water in nature. But did you know Mollies can even be found in the ocean, in full salt water, off the Florida Keys! Many snorkelers have seen playful black and sail fin Mollies at play in full salt water, though they are a tempting snack for natural salt water fish of many varieties so don’t expect to see many if you go looking next holiday.

Many salt water hobbyists have used the black Mollie as a first fish to “turn the tank and start the nitrogen cycle, but that is a story for another article!


For more information check out our Twitter, Facebook and Pinterest and feel free to ask questions by clicking here. If you encounter problems, come back to and I will do my best to help your aquarium thrive. I love fish and I want you to grow with the hobby. I want you to have all the information at your fingertips to create a happy healthy aquarium community for you and your family.