Despite their familiarity, mollies are much misunderstood fishes. They may be cheap, and they are certainly hardy and easy to breed when kept correctly, but in many ways they are not ideal fish for the community aquarium. Even defining ‘a molly’ is difficult, and things become even more complicated when trying to determine the ideal water conditions for these fish. One of the most common statements made about these fish is that they need to be kept in brackish water to do well; yet in the wild they are largely found in freshwater streams. So what is the truth about mollies?

Mollies and guppies are very closely related, both being members of the genus Poecilia. Mollies had previously been placed in their own genus, Mollienesia, and it was from this that the common name ‘molly’ was derived. The name Mollienesia was coined in honour of Francois Nicolas Mollien, a French politician of the late eighteen and early nineteenth centuries.

Practically all the mollies sold to aquarists are hybrids of a number of species found across Central America and the southern United States. Although it is common to describe aquarium fish as being Poecilia sphenops or whatever, in reality the genetics of all commercially produced fish is so hopelessly muddled that applying any sort of Latin name to them is completely pointless.

It is possible, however, to identify the original ancestors of today’s aquarium Molly, and it is important in as far as it reveals some valuable clues as to the optimal conditions under which these fish should be maintained. Poecilia mexicana and Poecilia sphenops are two rather similar species both known as shortfin mollies. They are both rather variable in terms of colour, though typically greenish-silver with patterns of olive, brown, yellow, and black spots. It is often said, though not with any overwhelmingly compelling evidence, that Poecilia mexicana in particular was the ‘rootstock’ from which most of the aquarium strains of molly were derived by careful crossing with other species of molly displaying desirable features.                  (more below….)


Two larger species are known as sail-fin mollies, Poecilia latipinna and Poecilia velifera. Though the males of all mollies have larger dorsal fins than the females, these two species have developed this attribute to a remarkable degree. On some males, the dorsal fin can be almost as long as the fish itself, and twice as tall as the fish is deep.

Needless to say, such fish are very impressive, especially when displaying. The dorsal fin serves two purposes. Firstly, it is an honest indicator of genetic fitness. A big dorsal fin costs energy to make and reduces swimming ability by increasing drag, so any male that reaches sexual maturity carrying one around must have good genes,  or it would have starved by now, or been eaten by a predator.

Females will therefore always select the males with the biggest dorsal fins, because they are assured of obtaining good genes for their offspring. The big dorsal can also be used to intimidate rival males: bigger fish tend to have bigger dorsal fins, so fin size makes it easy for males to judge one another’s size. And as there is no point to a smaller fish fighting – he’s going to lose – the comparing of fins allows each fish to decide whether to press their case, or rather swim away.

A few other species may also have played some role in the development of the aquarium molly strains, most notably Poecilia salvatoris, a species known as the liberty molly in the United States thanks to its brilliant red, white, and blue colouration. Although not commonly traded, this beautiful fish does turn up periodically, as does the brackish water shortfin molly Poecilia gillii and the remarkable swordtail sailfin molly Poecilia petenensis.

Besides the fact that aquarium mollies aren’t any one species, what muddies the waters even further is the remarkable adaptability that mollies show in terms of environmental adaptation. There really isn’t any one definitive ‘molly habitat’. Poecilia mexicana is found, for example, even in and around caves, where the waters are dark and rich in calcium sulphate. These cave-dwelling populations of Poecilia mexicana are pink and have relatively small eyes, but have highly developed taste receptors on the head and an enlarged lateral line system to compensate for the lack of light. Other mollies are found in estuaries and around mangroves, being well able to live in full-strength seawater without problems.

For the most part though, mollies are inhabitants of freshwater streams flowing across coastal plains. The water chemistry is typically hard and alkaline, pH 7.5 to 8.0 and hardness 15-30 dH.  Although water temperature may vary depending on geographical location, most mollies seem prefer warm water environments and are most common where the water temperature is around 25-28°C. These streams aren’t saline though, and while mollies certainly are found in brackish waters, that isn’t their primary habitat.


Mollies have been transported around the world and released into all kinds of environments. There are, for example, populations of mollies established in the marineGulfofThailand. Mollies have also become established in theUnited Statesin freshwater rivers and lakes far inland, where they can pose a serious threat to the survival of native fishes. In these cases, the mollies concerned are hybrids very like those kept by aquarists. In fact, in many cases they were descended from aquarium fish turned loose by careless aquarists and fish breeders. Because they are so similar to the aquarium fish, these feral mollies demonstrate quite clearly that while mollies can live in brackish and salt waters, they are just as able to survive in freshwater environments as well.

So, herewith comes the big question: To salt, or not to salt? If mollies can live in freshwater in the wild, why do so many experienced aquarists recommend keeping them in brackish water? To understand this, it is important to realise that marine salt mix does more than simply raise the salinity.

Marine salt mix contains table salt plus a huge variety of other mineral salts, including a large quantity of salts that raise pH and increase hardness. The addition of marine salt to molly aquarium provides mollies with water chemistry much closer to that which they prefer and also acts as a buffer, inhibiting any subsequent water chemistry changes. An aquarist adding a certain amount of marine salt to the molly aquarium is effectively guaranteeing the correct water conditions without any need to mess about with pH buffers or water hardening agents. Tonic or table salt cannot do this, because it contains nothing but sodium chloride, a chemical that doesn’t modify pH or hardness at all and should not be used for this purpose.

Using plain table salt – sodium chloride – is old-fashioned, but it did have some useful function way back when: It dramatically reduced the toxicity of nitrite and nitrate, a purpose for which it was put to extensive use in the early days of the hobby. Because filters back then were less efficient and water changes were performed less often (based on the old fashioned theory that ‘old water’ was better) the addition of small amounts of table salt to freshwater aquaria actually did some good. Repackaged table salt, known as ‘tonic salt’, is still available to aquarists to this day, though it has no real purpose in the modern hobby.

Excellent filtration and more water changes are the best way to deal with poor water quality, and really applies to all aquarium fish! But mollies do appear to be peculiarly sensitive to nitrite and nitrate, despite their widespread sale as beginners’ fish. In saline water, the toxicity of these compounds is so much less that the fish come to no harm; in fact, mollies have been used for decades to mature marine and brackish water aquaria. But if they are to do well in freshwater tanks, they need excellent water quality.

 Salt, whether marine mix or table salt, when used in sufficient quantities,  is also an effective anti-parasite and antifungal medication. But the widely known one teaspoon-per-gallon quantity, though often recommended, will have at most a marginal effect compared with commercial whitespot or fungus remedies. But once the salinity gets to a fairly high level, around 20-25% the salinity of normal seawater (SG 1.004-1.006) then most freshwater parasites find it very difficult to survive, and fungal infections tend to fade away quite rapidly. Mollies kept in around half-strength seawater (SG 1.012) will as good as never get infected with parasites because very few, if any, brackish water parasites have managed to become established in the aquarium hobby.

In contrast, mollies kept in freshwater aquaria are extraordinarily prone to a number of diseases, including whitespot, fungus, finrot, and ‘the shimmies’ (a neurological disorder that manifests itself as an inability for the fish to swim properly, instead the fish can only tread water, rocking from side to side). Though the absence of salt doesn’t cause these problems, adding salt is certainly one way to deal with them.  Low temperatures are likely a factor in mollies getting sick, because a fish’s immune system is optimised to work within a certain range of temperatures. The aquarium standard of 25°C is at the low end of what mollies enjoy, and it is probably this, coupled with poor water quality, that makes them so disease-prone in the average community tank.

Another problem with mollies is that they are more herbivorous than carnivorous, and evolution has primed them for grazing rather than gorging. Compared with standard tropical fish like tetras and barbs, mollies have very distinctive jaws featuring a unique set of hinges that allow for extreme protrusion of the premaxilla (the front part of the upper jaw). Both jaws are equipped with a single row of sharp teeth, and by pressing the mouth against solid objects and then extending and withdrawing the jaws, mollies can very efficiently scrape away diatoms, filamentous algae, and various other microorganisms. Algae are difficult to digest, but mollies have a long digestive tract that gives their digestive enzymes time to do their work. So although what they prefer to eat is poor in protein and difficult to digest, there is at least plenty of it, and they can feed more or less continuously during the daylight hours nibbling away at algae and decaying plant matter. If there is not sufficient algae in the aquarium, Mollies enjoy an occasional light feeding of finely chopped, lightly boiled spinach.

 Of course mollies do eat a certain amount of animal prey, particularly the larvae of insects such as mosquitoes and midges. But in the wild,  such foods are a supplement rather than a staple. As with many other herbivorous fishes, such as surgeonfish and Panaque catfish, when given nothing but foods rich in animal proteins, such as standard fish flake, the overall health of the fish will decline!  Experienced molly breeders have also found that the more algae and vegetables in the diet of the mothers, the bigger and healthier their broods. In short, the serious molly keeper should be providing them with algae and algae wafers, soft vegetables such as courgette, and vegetarian flake foods. Bloodworms and live daphnia make excellent treats, but they shouldn’t be used too often, once or twice a week at most.

If we pull all these issues and observations together, it is obvious that mollies are much more demanding fish than most aquarists assume. But the key things to get right can be summarised fairly easily: All mollies need very clean water with no nitrite and as little nitrate as possible. The water needs to be warm, hard and alkaline and they need the correct diet. Most aquaria are slightly too cool for mollies to really thrive, and the soft and acidic water the majority of aquarium fish enjoy is positively harmful to mollies over the long term. The addition of marine salt mix isn’t essential — but it will offset any lapses in water quality management and will raise the pH and hardness levels to where they should be. Table salt, or tonic salt, is perhaps better than nothing, but because it contains nothing to harden the water, it is far inferior to marine salt mix in terms of benefits. If you’re going to add salt, the exact amount doesn’t matter because mollies will accept anything from freshwater to full strength seawater, but in terms of benefits against cost, aiming for 20% seawater (SG 1.004) would be about right. At this salinity, your freshwater-adapted filter bacteria will not be harmed, and the amount of salt needed would not be very great, about 7.5grams per litre. It also pays to note that Mollies do best  in a group with at least 3 males and about twice as many females, or multiples of this ratio.


All of the above would seem to make mollies incompatible with community tropicals, and to some extent that is true. But, although things like tetras and Corydoras probably don’t have any place in the molly aquarium, there are plenty of fishes that will thrive in the warm, hard, alkaline water these fish need – you just need to choose the correct fish as tank mates. Almost all of the New World livebearers enjoy these conditions, and most will even tolerate small amounts of salt if necessary. Other freshwater fishes tolerant of such conditions include glassfish and wrestling halfbeaks. At SG 1.004, a nice variety of unusual brackish water fishes could be kept as well, including bumblebee gobies, Brachirus sp. soles, and orange chromides. The violet goby Gobioides broussonnetii would be an especially interesting addition, coming from the same parts of the world as the mollies and inhabiting the same sorts of waters

Of course, the serious molly aficionado appreciates that mollies are perfectly capable of being fascinating and rewarding animals kept all by themselves. Though sadly misunderstood, mollies remain wonderful fish well worth getting to know better.


Finally, a word or two on breeding mollies: Yet again, passed on wisdom often jars with actual reality. Mollies may seem easy to breed – they’re livebearers after all – but lots of people have tried and failed to rear broods of baby mollies. There is, of course, no real secret to breeding mollies. It is simply not to do so in community tanks, and mollies are far too large to be confined to breeding traps or nets.

The best way to breed mollies is actually very simple. Breed them in a dedicated breeding tank. Keep them warm, healthy, well fed (lots of greens), and don’t stress them. Stressed livebearers miscarry their broods, and though mollies are not quite as bad as halfbeaks in this regard, they’re pretty close. Confinement in a breeding trap is one sure way to stress a female molly, and another is persistent attention from an unwanted suitor. In a small aquarium, removing the male once the female is pregnant is a very good idea, as his efforts to mate can stress the female. How can you tell if a female is pregnant? Frankly, you can take it as a given once the males and females have been together for a few days. Also look out for an obvious swelling of the abdomen. Shortly before the fry are born, there is usually a dark patch around the vent known as the ‘gravid spot’.

Gestation varies somewhat, but is typically around a month. Litter sizes also vary. In  some cases over a hundred fry have been reported, with bigger females usually producing the largest broods. In the wild, the fry move into very shallow water where the adult mollies (and any predatory fishes) cannot go, so the females have never evolved any ‘maternal behaviour’ towards their offspring; as far as the female mollies are concerned, a baby molly is just a meal. As soon as the babies are born, remove the mother from the breeding tank, as the mothers sometimes may eat their own fry. (In community tanks, of course, the other fish hunt down and eat livebearer fry with remarkable enthusiasm.) Another way is to install masses of floating plants like Egeria, Cabomba, hornwort, or whatever’s handy. The baby fish will hide among the plants, and the aquarist can easily find and remove them to another aquarium where they can be reared safely. Molly fry grow very quickly, but they need plenty of food. Algae-based flake food six times a day is ideal, and there should certainly be some algae-covered stones in the aquarium to let them go snacking in between their proper meals. Throw in some floating plants too, as baby fish generally enjoy a bit of shade and appreciate the sense of security that the availability of hiding places gives them.

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