Description
The sailfin is the molly you buy for one reason: the sail. The adult male's dorsal fin is enormous, almost as tall as his own body, and when he spreads it in courtship or against another male the effect is matched by no other community fish.
It is also considerably larger than the common molly, up to 12 or 14 cm, and that shapes the tank: this is not a fish for 60 cm.
And here is the misunderstanding that causes the most disappointment: only adult males have the sail, and it takes months to develop. Shop juveniles look like ordinary mollies. Buy a group of youngsters expecting sails the next day and you will wait half a year — and with bad luck on sexes, you may never see one. Buy already-defined males if the sail is what you are after.
Like its short-finned cousin, it needs hard, alkaline water. In its case, even more so.
Behavior
Peaceful despite its size, and surprisingly calm for how showy it is.
The best part of keeping it is the dorsal display: males open the fin fully when courting females and when sizing each other up, turning sideways. It is pure showmanship with no physical aggression, and it is the reason people keep this species. That behaviour only appears where there is competition, so with a single male you miss half of it.
- Two or three females per male, as across the family.
- Two or more males is what triggers the displays, but it demands a genuinely long tank.
It lives well with common molly, swordtail, platy, corydoras, bristlenose and nerite snails: the usual hard-water crowd.
Avoid housing it with soft, acidic water fish, on parameter grounds; nippers such as tiger barbs, which ruin the sail and it takes a very long time to regrow; and small tanks, which is not a tank-mate problem but a space one.
With shrimp: adults yes, fry no.
It needs length to display comfortably. A metre is a reasonable minimum, and not for the litres but because a fish that displays sideways needs distance to do it.
Habitat
Poecilia latipinna lives in marshes, estuaries and coastal waters of the southeastern United States and the Gulf of Mexico. It moves naturally between very hard fresh water and brackish, depending on tide and season.
That coastal origin explains why it is even more demanding on hardness than the common molly. In soft water it does not cope: the characteristic shivering appears, then infections, then a death usually blamed on bad luck.
As with its cousin, salt is not compulsory as long as the water is genuinely hard and alkaline. Limestone, a calcareous substrate or hard tap water solve it without complications.
It also needs generous horizontal space and some flow. In its marshes it swims in open areas, and a tall, narrow tank makes it uncomfortable however much volume it holds.
Details
🌊 Water parameters
🐠 Minimum tank size
Breeding
A livebearer, with large broods — up to a hundred fry in adult females — every four to six weeks. The young are born swimming and independent from the first moment.
As across the family, the female stores sperm and keeps giving birth months after being separated from males.
What is specific to this species lies in the young males: they take a long time to define. They reach sexual maturity well before the dorsal starts growing, so for months you will have functional males that look like females. If you are selecting breeders on sail quality, be patient and do not cull anyone too early.
And as with the common molly: think before buying about what you will do with the broods.
Diet
Omnivorous with an even stronger herbivorous component than the common molly. In its marshes it grazes algae and plant detritus practically all day.
- Staple: flake or granules high in plant content.
- Plant matter: spirulina, nori, blanched courgette and peas, consistently rather than occasionally.
- Protein: frozen daphnia or brine shrimp, twice a week at most.
- Fry: the same food crushed.
Here diet has a visible consequence it does not have in other fish: the sail is built from what it eats. A poorly fed young male develops a smaller, duller dorsal, and that cannot be corrected later. If you bought the species for the fin, constant plant matter is not optional.
Like the common molly, it helps keep tank algae in check.
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