Common molly

Water parameters

Tolerated range for this species. Aim for the middle of each band rather than the extremes.

Tank and habitat

Substrate: any.

Behavior

Plant interaction: eats soft plants.

Feeding

Omnivore with a strong preference for vegetable matter. In the wild the protruding flat-faced jaws function as scrapers that rasp algae and biofilm from rock and plant surfaces, and aquarium fish continue this behaviour through the day. Diet needs to include real vegetable content: spirulina or vegetable-based flake or pellets as a staple, plus blanched zucchini, blanched spinach, blanched peas, and softened cucumber rotated through the week. Frozen bloodworm, brine shrimp, and daphnia are fine occasional supplements but should not dominate the diet. Underfed-on-vegetables mollies will strip soft-leaved aquarium plants (cabomba, hygrophila, hornwort tips), eat new shoots, and pull biofilm off hardscape aggressively. Feed two or three small portions a day. Most ornamental varieties (black molly, dalmatian, gold, lyretail) feed identically; the malformed balloon strain often has trouble eating efficiently because of its compressed body.

Compatibility

• Hard, alkaline water is non-negotiable. Mollies do not thrive in soft acidic community setups built for tetras, rasboras, or apistos; the parameter mismatch causes chronic stress
• Best paired with other hard-water species: platies, swordtails, endler's livebearers, larger guppies, gouramis that tolerate hard water, and corydoras (which handle harder water than most South American tetras)
• Tolerates and breeds well in slightly brackish water (around 3 to 6 PSU). A peer-reviewed study showed peak breeding success at 6 PSU. Fully marine setups are too much, but a teaspoon of marine salt per 4 litres suits mollies and most livebearer tankmates
• Keep one male per three or more females. Males harass single females to the point of stress and fin damage
• Soft-leaved planted tanks (cabomba, hygrophila, hornwort tips) get eaten; pair with hard-leaved species (vallisneria, anubias, java fern, amazon swords) for visible plants that survive
• All Neocaridina-shrimp adults are safe with adult mollies, but shrimplets get eaten by the much larger livebearer

Origin and habitat

Valenciennes described P. sphenops in 1846 as a Mexican poeciliid. The species was historically treated as a wide-ranging Central American molly extending from Mexico to Venezuela, but current taxonomy recognises that the older 'P. sphenops' is actually a species complex of around 13 distinct taxa, and P. sphenops itself is now restricted to Mexico, Guatemala, and Honduras. The complex is split between a Pacific-slope group (P. sphenops sensu stricto and relatives) and an Atlantic-slope group (P. mexicana and relatives). The two species look very similar and are frequently confused; the key diagnostic is dentition, with P. sphenops carrying tricuspid inner jaw teeth and P. mexicana carrying unicuspid teeth. Wild P. sphenops reaches about 8.4 cm SL. The trade name 'molly' (and particularly 'common' or 'short-fin' molly) is used loosely and most aquarium-trade mollies are actually hybrids: P. sphenops crosses with the sailfin molly (P. latipinna) and the Yucatan sailfin (P. velifera) are common, and the familiar fully-black 'black molly' or 'midnight molly' is itself originally a selectively bred melanistic form of P. sphenops which is now usually a P. sphenops × P. latipinna hybrid in the trade. Other selectively bred forms include dalmatian, gold, silver, lyretail, and the controversial 'balloon' variety; balloon mollies have a deliberately deformed, foreshortened and rounded body and suffer disproportionately from swim-bladder and digestive problems, with reduced lifespan. The species naturally occurs in fresh water streams and ponds and in coastal brackish water near estuaries and mangroves, and is genuinely euryhaline: peer-reviewed work by Khairnar et al. (black molly P. sphenops, AACL Bioflux 7(1):8-14) demonstrated 100 percent survival at 0 to 8 PSU with the best breeding performance at 6 PSU. The fish does not require salt in the aquarium but does better in hard, alkaline water (pH 7.5 to 8.5, GH 12 to 25) than in soft acidic conditions, and lightly salted water (1 to 1.5 teaspoons of marine salt per gallon, around 3 PSU) suits the species and often resolves disease in stressed imports. Mass commercial production in the Far East and Eastern Europe has accumulated decades of inbreeding, so trade stock is often genetically weaker than the species would be from wild collection; black mollies in particular are noticeably more disease-prone. Released and escaped mollies are established outside the native range in the US, Trinidad, Singapore, Hungary, Italy, Japan, and Brazil, with thermal-spring populations in Italy, Japan, and Montana.

Breeding

Livebearer that reproduces with no real intervention from the keeper. Males have a modified anal fin (gonopodium) used for internal fertilisation, and a single mating can produce several broods because females store sperm. After a gestation of roughly four to six weeks, the female releases 20 to 60 fully-formed, free-swimming fry, depending on female size and condition. The fry are large enough to take crushed flake immediately and grow quickly in a tank with reasonable water quality. Adults will eat fry given the chance, so densely planted areas, floating plants, or a separated breeding/nursery container produce noticeably more surviving fry. Population control is the real management problem: a single female produces broods every four to six weeks for most of her life, and a starting trio becomes dozens to hundreds of fish within a year if nothing is removed. The best strategy is to keep one male per three or more females (males harass females otherwise) and to plan upfront for either a predator tankmate that takes excess fry, a culling protocol, or a network of homes that will take the surplus.

Common problems

Shimmying (rocking the body side to side without forward motion) is the classic molly stress signal and almost always indicates a water-quality problem: low minerals, soft or acidic water, ammonia or nitrite spike, or sudden temperature shift. Mollies prefer harder and more alkaline water (pH 7.5 to 8.5, GH 12 to 25 dGH) than most tropical community fish, and the most common cause of chronic illness is housing them in soft acidic 'standard community' conditions. Columnaris and fin rot are common in stressed imports, particularly fish moving from brackish farm conditions into freshwater retail tanks; aquarium salt at low dose (about 1 tablespoon per 10 litres) helps when introducing new fish. Velvet, ich, and bacterial infections follow whenever the species is kept marginal. The balloon strain carries the constant background risk of swim-bladder failure because of the deformed body cavity. Overbreeding is a real long-term problem: a single trio in a stable tank can flood the aquarium within months. Male aggression and harassment of females causes fin damage and exhaustion if the gender ratio is wrong (one male to three or more females is the safe minimum). Plant damage in underplanted or under-fed tanks is the other complaint; soft-leaved species are eaten and biofilm is stripped, so feeding adequate vegetable matter and keeping hardy plant species solves it.

Bioload

Further reading

• How to cycle a fish tank
• Stocking ideas by tank size