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TFH Classic: Salt and Other Sailfin Secrets

by TFH Magazine on November 21, 2012 at 9:25 am

Sailfin molly. Photograph by Mark Smith.

By Robert Bock

From TFH December 2002

There are many controversies surrounding mollies. In this article, the past president of the North American Native Fishes Association reports his investigations and findings.


Want to start an argument among molly enthusiasts? Just bring up the subject of salt. One camp will say that you don’t need salt in the water to raise big, beautiful, sailfin mollies. Salt just allows lazy aquarists to compensate for dirty water, they say. It simply kills off the opportunistic bacteria and other parasites that victimize fish whose immune systems are stressed by the high nitrogen levels found in poorly maintained tanks. Keep fish densities low, do regular water changes, and you’ll have no problems raising big, beautiful sailfins that will be the envy of your fish club.

Not so, says the salt camp. Everyone knows that the biggest, most beautiful sailfin mollies come from saltwater habitats. Raising sailfins in fresh water produces, at best, small, stunted offspring that bear only faint resemblance to their wild forebears.

Who’s right? After keeping these beautiful fish for a few years, and conducting a little at-home research, I’ve come to the conclusion that they both are. For starters, sailfin mollies consist of three species, two of which are the most important for aquarists. Poecilia latipinna is found from South Carolina through Florida, along the Gulf Coast, and into Mexico. In the more southerly part of its range, P. latipinna is replaced by P. velifera, found along the coastal waters of the Yucatan Peninsula in Mexico.

These species resemble each other greatly, the major difference being that P. velifera usually has a larger body size, and the males have a larger dorsal fin than do males of P. latipinna. Male P. velifera have from 16 to 19 dorsal rays; male P. latipinna have only 12 to 14 rays. Both species inhabit brackish and freshwater habitats along the coast.

Commercially raised sailfin mollies come in an assortment of colors: black, white, mottled, yellow, and orange. In recent years, my favorite variation, the wild type “blue” sailfin molly, has started showing up in aquarium stores. The genetic pedigrees of these store-bought mollies vary greatly. They may be descended from P. latipinna or P. velifera, or hybrids of the two species, or from a hybrid of one or both species and some other related poeciliid species. Because of their mixed parentage, some of these mollies may not produce offspring capable of surviving on their own.

The Quest

I began my quest to raise the biggest, best sailfin mollies I could about four years ago. My friend, B. G. Granier, sent me a batch of wild-caught sailfin mollies, P. latipinna, from near New Orleans, Louisiana.  They were among the most beautiful creatures ever to grace my fishroom. There were about 10 males in the group, all of them shimmering azure and silver, each trimmed with orange on the face, dorsal, and tail fins.

Some people believe salt is needed to raise big, beautiful sailfin mollies, while others do not. Photograph from TFH Archives.

One by one, I watched them slip away. It began with a slight wobble. Their condition grew worse, until they could barely hold steady in the water. Eventually, they developed a downward curving spine.

At first I thought it was a contagious disease, brought in from the wild. I treated them with a variety of medicinal products. Nothing worked. One after another, they began to die. I wondered if it would be best to swear off this beautiful species for good before I killed any more. B. B. had warned me that they might need a little salt in their water to do well. But what I’d read said that salt was unnecessary, so I hadn’t added any.

The remaining male lingered in a quarantine tank. I pondered his curved spine and how much he reminded me of an older woman with osteoporosis. Perhaps he didn’t have a contagious disease. I remembered how some women develop osteoporosis later in life when they lack calcium in their diets during their reproductive years. Maybe this fish had a nutritional deficiency resulting from a lack of calcium.


I didn’t have anything left to lose. If I didn’t try something soon, the fish would probably be dead in a few days. So I dumped a tablespoon of garden limestone into the water. Limestone, I reasoned, is largely calcium carbonate and might provide a nutritional boost to help the fish hang on.

Soon after, the wobbling stopped. Although his spine never straightened out, this male lived on for many more months. Limestone, I soon learned, forms clumps and really doesn’t dissolve very well in water. Eventually, I began using another source of calcium carbonate—the Rift Lake salt favored by cichlid keepers. Within a couple of years I had four sailfin breeding tanks and an Everglades biotope tank crowded with sailfin mollies. Wild sailfin females are a drab gray, resembling large female guppies. To add a little color to the tank, I began keeping orange and mottled sailfins I bought at the aquarium store.

According to the books and articles I’d read, sailfin mollies inhabit salt water, brackish water, and fresh water. In an aquarium they would do well under any of these conditions. But what I’d read hadn’t explained why the fish seemed to need a calcium-rich environment.

Salt or Calcium?

A short time later, I happened upon the work of Joel Trexler, an ecologist at Florida International University in Miami. Dr. Trexler studies the effects of environmental influences on evolution. One of his major research interests is on how environment influences the development of sailfin mollies. I told Dr. Trexler about how adding garden limestone to the tank saved my sailfins from certain doom. He explained that sailfins are more suited to life in salt water than in fresh. Specifically the gill structures of sailfin mollies have osmoregulatory systems more like those of marine species. Osmoregulation refers to a fish’s ability to maintain the balance between the sodium and other minerals inside its body and the sodium and mineral concentration of the water it lives in.

Many fishes, including sailfin mollies, can use calcium in place of sodium for maintaining their osmotic balance, he said. For sailfins, water with high carbonate hardness also explains the distribution of sailfin mollies, at least throughout the East Coast. At the top of their range, in South Carolina, P. latipinna are confined to the salt and brackish waters along the coast. In southern Florida, their range extends across the peninsula. Much of south Florida sits atop ancient coral beds, and so its water have fairly high carbonate hardness. Similarly, P. velifera are either found in saltwater habitats or in limestone springs, and the geology of the Yucatan has much in common with southern Florida.

Calcium carbonate works the same way as salt when added to the molly aquarium. Photograph by Mark Smith.

One caveat is worth inserting here. A few populations of sailfin mollies might be genetically adapted to survive without salt or calcium carbonate. Virtually all the mollies I’ve kept die if they aren’t kept in very hard water, with a pH of at least 7.8. However, Bill Allen, of the American Livebearer Association, has told me of a drainage ditch full of sailfin mollies in Shreveport, Louisiana, about 200 miles from the Gulf Coast. The mollies there are apparently thriving in a pH of about 7.0, with total dissolved solids of less than 100 parts per million.

To complicate the great salt debate even further, many aquarium strains of sailfin mollies may have resulted from crosses with other freshwater species. In an email, Trexler told me that long-time commercial fish breeder Ross Socolof had told him that the aquarium black molly strains were developed from crosses between P. velifera and the  freshwater species, P. sphenops. For this reason, P. sphenops genes may have provided some strains of commercially available sailfins with a greater tolerance to fresh water than their wild-caught cousins have. However, with one or two exceptions, the store-bought sailfins that I’ve kept have done better in sea water than they have in fresh.

Salt and Calcium?

Although sailfins need calcium carbonate to maintain their osmoregulatory system, I have a theory that—because of the hunched backs that my fish developed—they may also have a higher nutritional need for calcium than other fish do. The males’ large dorsal fins contain numerous elongated rays, which may require extra calcium to develop. Dietary calcium may be available to them in algae and microscopic crustaceans they eat, having been absorbed by the latter from the water they live in.

I should point out, though, that Trexler questions the need for extra calcium in the sailfins’ diet. Armored catfish, he said, come from a soft water environment with little calcium and don’t seem to need any extra calcium to develop their armored plating. Since no scientific studies have been done on sailfins and dietary calcium, however, there isn’t any evidence to prove either of us right or wrong.

The Data

The varied conditions that wild sailfin mollies live under provide a natural laboratory that Trexler’s group employs to study the effects of environment on an organism.

One of his group’s experiments, in particular, is relevant to aquarists. He and his coworkers collected sailfins from four different locations throughout Florida. They raised the offspring of these fish under a combination of different conditions to see how those conditions might affect the fish’s development.

The scientists raised the mollies individually in 5-gallon aquaria in water with three different concentrations marine salt, 2 ppt (parts per thousand), 12 ppt, or 20 ppt. The fish were kept at a temperature of either 74° or 84.2°F and were fed either the maximum amount of food the fastest-growing individuals could eat in one day, or half that amount. The scientists reared groups of fish in each of the 48 different combinations possible.

Before beginning the study, Trexler and his colleagues did some informal tests to determine if the fish actually needed salt. The researches learned that the mollies grew more slowly and were more likely to die from fungal infections in water that had no salt. In water with 2 ppt salt, virtually no fish died.

Surprisingly, the amount of food the fish were given—a good quality flake—didn’t vary enough to have an effect on their final size. The greatest size at maturity, however, was reached by fish kept at 84.2° in water with a salinity of 2 ppt. Although their gill structure suggests that sailfin mollies do benefit from some salt in the water, that amount doesn’t seem to be that high. It’s worth noting here, also, that Trexler’s study tested only three salt concentrations, so it’s impossible to generalize from them which salinity level mollies favor most. It does seem reasonable to conclude, however, that the ideal salt concentration for most sailfin mollies lies somewhere between 2 ppt and 12 ppt.

The lack of an influence for high salinity on body size, however, is puzzling, particularly in light of other, seemingly contradictory findings. In field studies, scientists have observed that the largest sailfins were found at higher salinities in their wild habitats. Similarly, in another experiment, Trexler and his colleagues found that, in outdoor cages, mollies grew larger in sea water than they did in fresh water. The researchers theorized that some unknown condition in the wild might favor the development of larger size at higher salinities. I’ve observed much the same thing in my own tanks. The sailfin mollies I raise in artificial sea water also seem to be larger than those I raise in fresh water.


I suspect that the unknown condition may be diet. According to Trexler, sailfin mollies are largely herbivores. Like mammalian herbivores, they have long, winding digestive tracts suitable for digesting plant matter. He and his coworkers have analyzed the stomach contents of wild sailfins and found them to contain mostly algae. So, too, mollies kept in aquariums spend nearly all their waking moments grazing on algae.

My hunch is that the faster growth I’ve seen in my saltwater aquariums and that Trexler and his group saw in their saltwater cages could be from marine microalgae. Marine microalgae may be more nutritious for sailfins than freshwater microalgae is. Alternatively, marine microalgae may simply be more palatable than freshwater microalgae. Mollies that I’ve kept in saltwater tubs in summer and in brightly lit saltwater aquariums in house always look well fed, even when I hadn’t fed them for a couple of weeks. In contrast, freshwater mollies I’ve kept under the same conditions in freshwater always look underfed. The freshwater microalgae seems to be harder than the saltwater microalgae, and so I think the fish simply can’t consume enough of it to meet their needs.

My hunch is that if you can feed mollies a sufficient amount of food in either hard fresh water or in 2 ppt sea water, they will grow as large as mollies kept in brightly lit seawater environments.

To test this, I’ve divided baby molly siblings from the same wild-caught mother into two groups. I’m rearing one in sea water and another in hard fresh water. Except for the salinity, I’m keeping both groups under nearly identical conditions. To make sure they get enough to eat, I allow both groups to graze nearly all day on cubes of high density food I make myself.

I feed homemade cubes because I don’t think it’s possible to raise large mollies on a diet of dry flake food, unless, perhaps, you can find the time to feed them a dozen or so times a day. Dry foods are less dense than pellets and frozen cubes, so feeding flakes two or three times a day, as most aquarists are able to do, simply won’t supply them with the comparable amount of nutrition they could get by grazing on marine microalgae all day.

To make my pellets, I begin by soaking high quality cichlid pellets until they’re water logged. As most aquarists know, fish can die if they eat too much dry food; the food takes on water inside the fish’s digestive tract, and the fish’s digestive organs burst. In contrast, fish can eat as much water-logged food as they like. I mix the pellets with an equal amount of unflavored gelatin other ingredients and turn on the blender until I have a dense slurry. I pour the slurry into empty plastic frozen fish food containers to form individual portion sizes, then freeze portions until I’m ready to use them.

Currently I’m still experimenting with the food mixture, and I’ll often add frozen brine shrimp, cooked peas, soaked flake, and marine macroalgae.

It’s been about a month since I’ve started keeping two groups of fish under these conditions. So far, the freshwater sailfins seem to be keeping pace with their saltwater siblings.

One promising idea that I hope to get around to eventually is to provide mollies kept in fresh water with algae grown in sea water. One way to do this would be by rotating algae-covered rocks from a seawater tank through the freshwater tank.

Growth Dimorphism

Another important lesson to learn from Trexler’s study of environmental conditions is how female body size is influenced more by environment that is male body size.

Although their environment clearly influenced the size of both sexes, female mollies reach their greatest size largely because of environmental conditions favorable to growth, whereas males reached their largest size because of the genes they had inherited. This finding confirms earlier studies, which suggest that male body size depends largely on a number of different genes. Male sailfins mature at various stages of development. When they reach sexual maturity, they stop growing.

The implication for hobbyists, though, is clear: If you want to raise a strain of large, colorful sailfins with big sails, it’s best to start with large male breeders. Keeping small males in the hopes that they will reach a large size, then, is fruitless. Instead, small males should be ruthlessly culled from the breeding tank, as their sons will also be small.

Personally, I select for female size and color. Although I prefer the coloration of wild-caught males to that of their store-bought cousins, female wild-type sailfins are rather drab. Right now, I’m trying to introduce some color into the females, both with store-bought orange varieties and with wild-type females having some faint orange coloration. If I come across an outstanding wild-type male, I’ll add him to my breeding stock, both to add new male genes and to prevent inbreeding. I don’t pay attention to whether my breeders are P. latipinna or P. velifera. Since both species interbreed freely, I’m not interested in maintaining a pure strain, just in producing an outstanding strain of blue males for the aquarium.


So, what about the Great Salt Debate? Mollies probably have the best chance for thriving if they are kept in water that is at least 2 ppt salt or has a pH of 7.8 or higher and is hard and alkaline. Although mollies will greedily take meaty foods such as insect larvae and brine shrimp, they are herbivores. No matter what  you feed them, it’s probably best to keep them with an abundance of algae to graze on.

Because they graze continually, sailfins produce large amounts of ammonia-containing wastes. For this reason, tank densities should be kept low. Low tank densities not only keep down levels of ammonia and other wastes, but also prevent stressful competition between males. The high pH that sailfins favor increases the ammonia’s toxicity, so would-be sailfin keepers need to take extra pains to keep ammonia levels down through frequent water changes, good filtration, and perhaps with the assistance of ammonia-absorbing, fast-growing plants.

For freshwater sailfin tanks, I grow Italian vals (Vallisneria spiralis), in salt water, the marine macroalgae Caulerpa taxifolia. Bright lighting will not only stimulate plant growth, but also that of macroalgae the mollies need to graze on. Although they’ve been in the hobby for many years, sailfin mollies are not a beginner’s fish. They do require some extra work, and some extra thought. But the beautiful colors of both store-bought and wild-caught fish, plus the magnificent sail stretching display of courting males, merit the attention of serious aquarists.


I’d like to thank Dr. Joel Trexler for his careful review of this manuscript. Articles on wild sailfin mollies are available on the website of the North American Native Fishes Association In addition, the American Livebearer Association has an excellent compilation of past articles on mollies


Posted in Uncategorized by TFH Magazine on November 21st, 2012 at 9:25 am.

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