Molly's the word!


Mollies have a mystique that sets them apart from other commonly traded livebearers, but with a reputation for being difficult to keep Neale Monks' A to Z guide into their weird and wonderful world is a must-read.

Mollies are really very adaptable fish, but to keep them well you really do need to know something about them.

A small molly is much bigger than the average guppy, and some of the biggest molly varieties are real giants. They are often described as more suited to brackish water systems than community tanks.

A is for Amazon

The Amazon molly is a very bizarre fish. The species is for all practical purposes female, but females mate with males from other molly species as and when the opportunity arises. Fertilisation doesn’t occur, and after mating all the male’s sperm does is trigger the development of the female’s eggs. The eggs have a complete set of genes already, identical to those of the female, so effectively the species clones itself with each female molly producing a batch of daughters genetically identical to herself after each mating!

B is for brackish water

Mollies are common in saline habitats, including brackish ditches and lagoons. Some may even be found in marine environments, albeit in very shallow water; for example, around the roots of mangroves or artificial habitats like harbours. A few species, like Poecilia gillii and Poecilia vandepolli, get bigger and show brighter colours in brackish water but you’re unlikely to see species such as these in tropical fish shops.

The mollies in aquarium shops are hybrids that do well in freshwater — provided other parameters are good, particularly water quality and hardness. Still, wise aquarists will stock the molly aquarium with salt-tolerant plants and tank mates, just in case a little salt is needed for good health.

C is for caves

One variety of Sailfin molly, Poecilia mexicana, inhabits the limestone caves of Mexico. Unlike a lot of cavefish, cave mollies have eyes but their habitat is completely dark so they need to find their food and one another by relying on their lateral line. Instead of algae they feed on sulphur bacteria, insect larvae and even bat droppings!

D is for dorsal fin

Male mollies often raise their dorsal fin when threatening rival males or trying to impress potential mates. Two species of molly have large sail-like dorsal fins, the Sailfin molly, Poecilia latipinna, and the Giant sailfin molly, Poecilia velifera. The size of the sailfin depends on environmental conditions; a spacious aquarium with good water quality and frequent water changes is needed for maximum development.

E is for extremes

Mollies are remarkable among livebearers for being adapted to a range of seemingly extreme conditions, including species that are tolerant of low oxygen levels, salinity levels above that of normal seawater and even high concentrations of hydrogen sulphide that would kill most other fish in minutes.

Ironically perhaps, some of these mollies are at risk of extinction. The Sulphur molly, Poecilia sulphuraria, is limited to a small area in Mexico where sulphur-rich springs feed into a number of streams and pools. As humans use more of the surrounding area for homes and farms, there’s less and less habitat for the Sulphur mollies. While they can live in ordinary freshwater, they can’t compete with regular mollies in such habitats so really do need their unique habitat to survive.

F is for food

While mollies are definitely omnivores that consume both animal and plant foods, the accent should always be on plant-based foodstuffs. Spirulina flake and pellets make good staples, while meaty things like bloodworms, though greedily consumed by all mollies, should be used sparingly: once or twice a week at most.

G is for Giant sailfin molly

Also known as Poecilia velifera, this species can get very big (around 15cm/5.9") and it’s the females that are the biggest. They are somewhat delicate and prone to stunting as well as opportunistic bacterial infections if aquarium conditions aren’t optimal. Needing very hard water to do well (at least 20ËšdH, pH 7.5), this species is probably easier to keep in slightly brackish water. Water quality needs to be good too, especially in freshwater aquaria.

H is for hybrids

When two species are crossbred, the resulting offspring are called hybrids. This easily happens with mollies, and hybrid mollies are probably very common in the trade particularly among the popular ‘fancy’ varieties. Dalmatian mollies likely inherited their spotted colouration from a spotted form of the Sailfin molly, Poecilia latipinna, while the Black molly is a hybrid based on Poecilia sphenops.

I is for islands

Many mollies are found in the Caribbean islands and several species are endemic to just one island or island group, for example, Poecilia elegans and Poecilia dominicensis to Hispaniola, and Poecilia vandepolli to the Lesser Antilles. Their tolerance of salty water allows mollies to colonise habitats closed to most freshwater fish, but that also makes them an invasive species if they’re accidentally introduced somewhere by people.

J is for jaws

Molly jaws can protrude forwards to make a sort of scraping tool ideal for rasping algae from plants and rocks.

While omnivorous in the wild and in captivity, it’s a good idea to ensure mollies get plenty of green foods to eat. In brightly-lit tanks there may be enough algae for them to eat, but this can be supplemented with Spirulina flake and other algae-based foods. Softened salads and veggies can be used too, with blanched lettuce and cooked spinach being particularly popular with most mollies. As mentioned before, minimise the use of meaty foods, especially rich foods like bloodworms. These would only be minor parts of their diet in the wild, and mollies tend to be healthier and live longer when given a predominantly plant-based diet.

K is for confusion (in Greek anyway)

Poecilia kykesis gets its name from a Greek word, 'Kykesis', that means 'a mixture' or 'confusion'. The name was a reference to the variety of mollies that had been lumped together as Poecilia petenensis.

In fact this species is one of several large Sailfin molly species that is greenish-silver in colour and inhabits freshwater and brackish water along the subtropical coastline of North America.

What is distinctive about Poecilia kykesis is the presence of a short extension to the lower lobe of the tail fin, similar to that seen on swordtails. On occasion these Swordtail mollies do turn up in the aquarium trade, and their care is similar to that of other Sailfin mollies. Needless to say, such similar and closely-related fish hybridise readily. So if you are lucky enough to get hold of Poecilia kykesis (or any other purebred molly species), it would be wise to keep them away from all other livebearer species so you can produce useful purebred offspring to pass on to other hobbyists.

L is for Liberty molly (picture above by George Farmer)

For many years regarded as a variant of Poecilia sphenops, this very pretty fish is now recognised as a distinct species, Poecilia salvatoris. Its common name refers to the similarity between the red, white and blue colouration on its fins and the colours of the American flag.

In aquaria Liberty mollies can be nippy towards other fish and aggressive towards one another. It’s best to keep them in a large group, with the females outnumbering the males.

M is for maximum size

Most aquarists will have read reports of female mollies that were 15cm/5.9” or more in length, but such giants seem rare in aquaria. Why is that?

One aspect is surely genetics. Breeders tend to be looking for colours rather than size, and fish farmers will make more money from fish that breed when young (and therefore relatively small) than those fish that develop more slowly (even if such fish eventually grow to be very big).

The end result is that farmed varieties of mollies are invariably smaller than their wild cousins. But diet and especially environment are likely to be factors too, and this can be observed in the wild as well as in aquaria. For example, Poecilia vandepolli from saltwater lagoons are substantially bigger than those in brackish water and twice as big as the ones in freshwater habitats!

N is for nitrate

Mollies seem to be peculiarly sensitive to nitrate, especially in freshwater aquaria.

Salt reduces the toxicity of nitrate (and nitrite), which may be one of the reasons that mollies often seem easier to keep in brackish water compared with freshwater.

As with many other nitrate-sensitive fish, such as dwarf cichlids, it is a good idea to aim for nitrate levels at or below 20 mg/l. Frequent water changes will help, as will lightly stocking the aquarium and feeding fish sparingly especially avoiding protein-rich foods such as bloodworms.

Fast-growing plants can remove nitrate between water changes, with floating plants being perhaps the easiest to grow and crop back as necessary.

O is for osmosis

When fish are placed in freshwater they tend to lose salts and gain water through a process called osmosis. The reverse of this happens in seawater; the fish gains salt but loses water. Uncontrolled, either can quickly lead to death; the fish either swell up with water in the first situation or become dehydrated in the second.

Mollies are euryhaline, meaning they can adjust their bodies to live in both fresh and saline conditions. In some cases they can live and breed perfectly well in full-strength seawater.

P is for Poecilia

There are at least 40 species currently included in the genus Poecilia, all broadly molly-like in shape and habit but varying in size and colouration.

Apart from the mollies, other members of this group include Poecilia reticulata, the guppy and Poecilia parae — a variable species notable for occurring in several distinct colour forms of which the Red melanzona morph is the one that is most commonly traded.

As they interbreed readily, it is important to keep Poecilia species separate if hybridisation is to be avoided.

Q is for quality

Water quality, that is! Mollies are a bit of a paradox when it comes to their suitability for new aquaria. In freshwater aquaria domesticated mollies can be delicate; in particular the fancy forms like Balloon mollies and such fish are best maintained in mature tanks with well-established biological filters.

On the other hand mollies can be quite tough when maintained in brackish or marine conditions, and historically Black mollies were often used to cycle new marine tanks. With the widespread use of live rock to mature marine tanks, cycling tanks with hardy fish of any kind isn’t recommended anymore.

R is for respiration

Like most fish, mollies respire by extracting oxygen from the water and pumping it through their gill cavities.

Mollies have the ability to put up with stagnant, oxygen-poor water longer than other livebearers. Mollies swim to the surface and pump the uppermost layer of water, which contains the most oxygen, across their gills. They can also increase the amount of haemoglobin in their blood to absorb oxygen from the water more effectively.

It’s not good to force mollies to work this way in aquaria, and if your fish spend a lot of time at the surface it could be a clue that circulation or aeration of the water isn’t adequate.

S is for sailfin

The males of several molly species have sail-like dorsal fins. Poecilia latipinna and Poecilia velifera probably have the biggest dorsal fins relative to body size, but even those on Poecilia petenensis and Poecilia kykesis are still substantial structures that dwarf those of the so-called Shortfin mollies like Poecilia sphenops.

The function of the sailfin is partly for display, but it’s also an encumbrance that makes life difficult for the male molly carrying it. So like the peacock’s tail, the molly’s dorsal fin is an honest advertisement of genetic fitness; a male molly must be strong and healthy if it has lived long enough to become sexually mature while carrying a giant fin that uses up nutrients, increases drag and makes swimming more difficult.

T is for tonic salt

Tonic salt is basically sodium chloride, i.e. cooking salt, but without the added iodine. When kept in soft, acidic conditions especially, mollies are prone to bacterial and fungal infections and small amounts of tonic salt helps to keep these at bay. A dosage of around one to two teaspoons per 10 l/2.1 gal of water is often recommended.

While useful, tonic salt used this way is inferior to a higher dosage of marine aquarium salt mix because it lacks the complete range of trace elements and buffering salts. If you’re setting up a planted brackish water aquarium, it’s better to use marine aquarium salt mix at a dose of about 4-6 g/litre (roughly a level teaspoon per litre) for a specific gravity of 1.002-1.003 at 25ËšC/77°F.

U is for unappreciated

The Balloon molly is a variety of molly that divides the hobby. It has a deformed spine and abdomen, but good quality specimens don’t seem to be any more disease-prone than any other domesticated, fancy molly variety. Wise aquarists keep them to themselves though, because they don’t compete well with standard mollies or indeed any other boisterous fish species.

V is for velifera

The biggest traded molly is the Giant sailfin molly, Poecilia velifera. Success with this species comes down to generous aquarium size, low-stocking density, excellent water quality, hard and alkaline water chemistry and an algae-rich diet. If these requirements are ignored, the species may still live but it often becomes stunted and the dorsal fin in particular may fail to reach its maximum size.

W is for water chemistry

Mollies need hard, alkaline water to do well and in the wild are rarely found in soft water. Aim to provide general hardness levels of 20ËšDH or higher and ensure the carbonate hardness is high as well, 10ËšKH or more, so that the pH stays steady between water changes, ideally around 7.5 to 8.5. Ideally keep mollies alone, but if you must keep them with other species choose ones with similar requirements.

The Brown hoplo catfish, Hoplosternum littorale, for example, would be a good choice as it comes from similar habitats and enjoys similar water conditions. It even tolerates slightly brackish water perfectly well.

X marks the spot

Many aquarists will be aware of the 'gravid spot', the dark patch that appears between the abdomen and the anal fin when female livebearers are carrying young. The patch itself is formed when the dark tissue of the uterus pushes against the thin muscle wall of the abdomen. Though clearly visible on small species like guppies, it isn’t so obvious on larger species like mollies, particularly ones with dark, intense or peppered colouration (like Black mollies or Dalmatian mollies).

Y is for Yucatan

This Mexican peninsula is a heartland of molly diversity, with several species being found there and often in the same streams and lagoons. Normally, closely-related species living together divide up their habitats so that they don’t compete with each other, and by living apart they don’t get the opportunity to hybridise. However, when scientists collected and examined four species of molly in Yucatan they found just four male hybrids, an astonishingly small number given they’d been collecting mollies there for more than thirty years! The authors suggest that each molly species has its own pre-mating behaviours that isolate it from other species; successful matings across species are consequently very rare.

Z is a good place to finish!

For such widely-traded fish, mollies are surprisingly poorly understood, but hopefully this feature has highlighted a few of the things that make them remarkable fish and well worth keeping.

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