Photo: Internet Archive Book Images (Public domain) — via Wikimedia Commons
Molly (Poecilia sphenops)
A bigger, bolder livebearer that grazes algae and loves hard water — the friendly workhorse of the community tank.
Will it live with a Molly?
We compare each fish against your molly on temperament, size, water parameters and swimming zone. Set your tank size and filter the results.
- Badis✅ CompatibleSemi-aggressive · 8 cm · Medium care · 22–28 °C (72–82 °F)
- Peaceful + Semi-aggressive, but with no direct clash here, and their water overlaps around 23–28 °C — no size, zone or temperament conflicts.
- Black Kuhli Loach✅ CompatiblePeaceful · 8 cm · Easy care · 23–28 °C (73–82 °F)
- Compatible on the things that matter: shared water near 23–28 °C, workable temperaments, and no predator-and-prey size gap.
- Bolivian Ram✅ CompatiblePeaceful · 8 cm · Medium care · 24–28 °C (75–82 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Brilliant Rasbora✅ CompatiblePeaceful · 9 cm · Easy care · 22–28 °C (72–82 °F)
- Both are peaceful, and their water overlaps around 23–28 °C — no size, zone or temperament conflicts.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Brilliant Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Burmese Loach✅ CompatiblePeaceful · 9 cm · Easy care · 22–28 °C (72–82 °F)
- Both are peaceful, and their water overlaps around 23–28 °C — no size, zone or temperament conflicts.
- Celebes Rainbowfish✅ CompatiblePeaceful · 7 cm · Medium care · 22–28 °C (72–82 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Celebes Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Cockatoo Dwarf Cichlid✅ CompatibleSemi-aggressive · 8 cm · Medium care · 24–28 °C (75–82 °F)
- Peaceful + Semi-aggressive, but with no direct clash here, and their water overlaps around 24–28 °C — no size, zone or temperament conflicts.
- Costa's Tetra✅ CompatiblePeaceful · 7 cm · Easy care · 22–28 °C (72–82 °F)
- Compatible on the things that matter: shared water near 23–28 °C, workable temperaments, and no predator-and-prey size gap.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Costa's Tetra in a shoal of 6+ or it gets stressed and nippy.
- Croaking Gourami✅ CompatiblePeaceful · 7 cm · Easy care · 24–28 °C (75–82 °F)
- Compatible on the things that matter: shared water near 24–28 °C, workable temperaments, and no predator-and-prey size gap.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Dwarf Gourami✅ CompatibleSemi-aggressive · 8 cm · Medium care · 24–28 °C (75–82 °F)
- Peaceful + Semi-aggressive, but with no direct clash here, and their water overlaps around 24–28 °C — no size, zone or temperament conflicts.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Giant Kuhli Loach✅ CompatiblePeaceful · 12 cm · Easy care · 24–30 °C (75–86 °F)
- Both are peaceful, and their water overlaps around 24–28 °C — no size, zone or temperament conflicts.
- Gold Barb✅ CompatiblePeaceful · 7.5 cm · Easy care · 18–26 °C (64–79 °F)
- Both are peaceful, and their water overlaps around 23–26 °C — no size, zone or temperament conflicts.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Gold Barb in a shoal of 6+ or it gets stressed and nippy.
- Goldeneye Dwarf Cichlid✅ CompatibleSemi-aggressive · 8 cm · Easy care · 24–28 °C (75–82 °F)
- Compatible on the things that matter: shared water near 24–28 °C, workable temperaments, and no predator-and-prey size gap.
- Leopard Frog Pleco✅ CompatiblePeaceful · 9 cm · Medium care · 25–30 °C (77–86 °F)
- Both are peaceful, and their water overlaps around 25–28 °C — no size, zone or temperament conflicts.
- Mahachai Betta✅ CompatibleAggressive · 7 cm · Medium care · 24–30 °C (75–86 °F)
- Peaceful + Aggressive, but with no direct clash here, and their water overlaps around 24–28 °C — no size, zone or temperament conflicts.
- Murray River Rainbowfish✅ CompatiblePeaceful · 11 cm · Easy care · 15–26 °C (59–79 °F)
- Both are peaceful, and their water overlaps around 23–26 °C — no size, zone or temperament conflicts.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Murray River Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Peacock Gudgeon✅ CompatiblePeaceful · 7 cm · Medium care · 22–28 °C (72–82 °F)
- Compatible on the things that matter: shared water near 23–28 °C, workable temperaments, and no predator-and-prey size gap.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Peppered Corydoras✅ CompatiblePeaceful · 7 cm · Easy care · 18–26 °C (64–79 °F)
- Both are peaceful, and their water overlaps around 23–26 °C — no size, zone or temperament conflicts.
- Keep Peppered Corydoras in a shoal of 6+ or it gets stressed and nippy.
- Roundtail Paradise Fish✅ CompatibleSemi-aggressive · 7 cm · Easy care · 10–26 °C (50–79 °F)
- Peaceful + Semi-aggressive, but with no direct clash here; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Splashing Tetra✅ CompatiblePeaceful · 8 cm · Medium care · 22–28 °C (72–82 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Keep Splashing Tetra in a shoal of 6+ or it gets stressed and nippy.
- Spotted Corydoras✅ CompatiblePeaceful · 7 cm · Easy care · 22–27 °C (72–81 °F)
- Both are peaceful, and their water overlaps around 23–27 °C — no size, zone or temperament conflicts.
- Keep Spotted Corydoras in a shoal of 6+ or it gets stressed and nippy.
- Spotted Rubbernose Pleco✅ CompatiblePeaceful · 12 cm · Medium care · 20–26 °C (68–79 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Sumo Loach✅ CompatibleSemi-aggressive · 7 cm · Medium care · 22–26 °C (72–79 °F)
- Compatible on the things that matter: shared water near 23–26 °C, workable temperaments, and no predator-and-prey size gap.
- Thick-lipped Gourami✅ CompatiblePeaceful · 9 cm · Easy care · 22–28 °C (72–82 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Afra Cichlid⚠️ With cautionSemi-aggressive · 10 cm · Medium care · 24–28 °C (75–82 °F)
- Afra Cichlid is semi-aggressive and may chase or nip the smaller Molly — plant heavily and break up sight lines.
- Your 75 L tank is below the ~150 L this pairing really wants — crowding raises aggression.
- Keep Afra Cichlid in a shoal of 6+ or it gets stressed and nippy.
- Bearded Corydoras⚠️ With cautionPeaceful · 10 cm · Medium care · 18–24 °C (64–75 °F)
- Your 75 L tank is below the ~80 L this pairing really wants — crowding raises aggression.
- Keep Bearded Corydoras in a shoal of 6+ or it gets stressed and nippy.
- Clown Rasbora⚠️ With cautionPeaceful · 10 cm · Medium care · 23–28 °C (73–82 °F)
- Your 75 L tank is below the ~110 L this pairing really wants — crowding raises aggression.
- Keep Clown Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Convict Cichlid⚠️ With cautionAggressive · 10 cm · Easy care · 21–28 °C (70–82 °F)
- Expect Convict Cichlid to harass Molly at times; give dense cover and watch them at feeding.
- Your 75 L tank is below the ~110 L this pairing really wants — crowding raises aggression.
- Daffodil Cichlid⚠️ With cautionSemi-aggressive · 10 cm · Medium care · 23–27 °C (73–81 °F)
- Daffodil Cichlid is semi-aggressive and may chase or nip the smaller Molly — plant heavily and break up sight lines.
- Your 75 L tank is below the ~120 L this pairing really wants — crowding raises aggression.
- Electric Yellow Cichlid⚠️ With cautionSemi-aggressive · 10 cm · Medium care · 23–26 °C (73–79 °F)
- Expect Electric Yellow Cichlid to harass Molly at times; give dense cover and watch them at feeding.
- Your 75 L tank is below the ~200 L this pairing really wants — crowding raises aggression.
- Giant Danio⚠️ With cautionPeaceful · 10 cm · Easy care · 20–27 °C (68–81 °F)
- Your 75 L tank is below the ~110 L this pairing really wants — crowding raises aggression.
- Keep Giant Danio in a shoal of 6+ or it gets stressed and nippy.
- Golden Wonder Killifish⚠️ With cautionSemi-aggressive · 10 cm · Easy care · 22–28 °C (72–82 °F)
- Expect Golden Wonder Killifish to harass Molly at times; give dense cover and watch them at feeding.
- Johanni Cichlid⚠️ With cautionAggressive · 10 cm · Medium care · 24–28 °C (75–82 °F)
- Expect Johanni Cichlid to harass Molly at times; give dense cover and watch them at feeding.
- Your 75 L tank is below the ~200 L this pairing really wants — crowding raises aggression.
- Keep Johanni Cichlid in a shoal of 6+ or it gets stressed and nippy.
- Keyhole Cichlid⚠️ With cautionPeaceful · 10 cm · Easy care · 22–28 °C (72–82 °F)
- Your 75 L tank is below the ~80 L this pairing really wants — crowding raises aggression.
- Kribensis⚠️ With cautionSemi-aggressive · 10 cm · Easy care · 24–27 °C (75–81 °F)
- Kribensis is semi-aggressive and may chase or nip the smaller Molly — plant heavily and break up sight lines.
- Kuhli Loach⚠️ With cautionPeaceful · 10 cm · Easy care · 24–30 °C (75–86 °F)
- Water hardness preferences differ (Molly 10–25 vs Kuhli Loach 1–8 dGH).
- Paradise Fish⚠️ With cautionAggressive · 10 cm · Medium care · 16–26 °C (61–79 °F)
- Expect Paradise Fish to harass Molly at times; give dense cover and watch them at feeding.
- Your 75 L tank is below the ~80 L this pairing really wants — crowding raises aggression.
- Porthole Catfish⚠️ With cautionPeaceful · 10 cm · Easy care · 22–28 °C (72–82 °F)
- Your 75 L tank is below the ~80 L this pairing really wants — crowding raises aggression.
- Rosy Barb⚠️ With cautionSemi-aggressive · 10 cm · Easy care · 18–26 °C (64–79 °F)
- Rosy Barb is semi-aggressive and may chase or nip the smaller Molly — plant heavily and break up sight lines.
- Keep Rosy Barb in a shoal of 6+ or it gets stressed and nippy.
- Rusty Cichlid⚠️ With cautionSemi-aggressive · 10 cm · Medium care · 24–28 °C (75–82 °F)
- Rusty Cichlid is semi-aggressive and may chase or nip the smaller Molly — plant heavily and break up sight lines.
- Your 75 L tank is below the ~150 L this pairing really wants — crowding raises aggression.
- Alligator Gar⛔ Not recommendedAggressive · 250 cm · Hard care · 24–28 °C (75–82 °F)
- Size gap is too large (250 vs 10 cm): Alligator Gar will treat Molly as food.
- Alligator Gar clearly outsizes Molly and is aggressive; risky unless the tank is big and well-planted.
- Your 75 L tank is below the ~3785 L this pairing really wants — crowding raises aggression.
- Clown Knifefish⛔ Not recommendedAggressive · 90 cm · Hard care · 24–28 °C (75–82 °F)
- Clown Knifefish (90 cm) is big enough to swallow the 10 cm Molly whole.
- Clown Knifefish clearly outsizes Molly and is aggressive; risky unless the tank is big and well-planted.
- Your 75 L tank is below the ~750 L this pairing really wants — crowding raises aggression.
- Fire Eel⛔ Not recommendedSemi-aggressive · 100 cm · Medium care · 24–28 °C (75–82 °F)
- Fire Eel (100 cm) is big enough to swallow the 10 cm Molly whole.
- Expect Fire Eel to harass Molly at times; give dense cover and watch them at feeding.
- Your 75 L tank is below the ~380 L this pairing really wants — crowding raises aggression.
- Koi⛔ Not recommendedPeaceful · 90 cm · Medium care · 4–28 °C (39–82 °F)
- Size gap is too large (90 vs 10 cm): Koi will treat Molly as food.
- Your 75 L tank is below the ~3800 L this pairing really wants — crowding raises aggression.
- Redtail Catfish⛔ Not recommendedAggressive · 120 cm · Hard care · 24–27 °C (75–81 °F)
- Size gap is too large (120 vs 10 cm): Redtail Catfish will treat Molly as food.
- Redtail Catfish clearly outsizes Molly and is aggressive; risky unless the tank is big and well-planted.
- Your 75 L tank is below the ~5700 L this pairing really wants — crowding raises aggression.
- Spotted Gar⛔ Not recommendedAggressive · 90 cm · Hard care · 18–26 °C (64–79 °F)
- Size gap is too large (90 vs 10 cm): Spotted Gar will treat Molly as food.
- Spotted Gar is aggressive and may chase or nip the smaller Molly — plant heavily and break up sight lines.
- Your 75 L tank is below the ~600 L this pairing really wants — crowding raises aggression.
- Wels Catfish⛔ Not recommendedAggressive · 300 cm · Hard care · 15–25 °C (59–77 °F)
- Size gap is too large (300 vs 10 cm): Wels Catfish will treat Molly as food.
- Wels Catfish is aggressive and may chase or nip the smaller Molly — plant heavily and break up sight lines.
- Your 75 L tank is below the ~20000 L this pairing really wants — crowding raises aggression.
- Wolf Cichlid⛔ Not recommendedAggressive · 72 cm · Hard care · 24–30 °C (75–86 °F)
- Size gap is too large (72 vs 10 cm): Wolf Cichlid will treat Molly as food.
- Wolf Cichlid is aggressive and may chase or nip the smaller Molly — plant heavily and break up sight lines.
- Your 75 L tank is below the ~760 L this pairing really wants — crowding raises aggression.
Compatibility is computed from each species' care data — a strong starting point, not a guarantee. Individual temperament varies, so always introduce new fish slowly and watch them.
Molly care specs
- Care level
- Easy
- Breeding
- Easy
- Max size
- 10 cm (3.9 in)
- Min tank size
- 75 L (19.8 gal)
- Temperature
- 23–28 °C (73–82 °F)
- pH
- 7–8.2
- Hardness
- 10–25 dGH
- Lifespan
- 3–5 years
- Diet
- Omnivore
- Swim level
- Middle
- Group size
- 3+ (shoaling)
- Family
- Poeciliidae
- Origin
- Central America — coastal Mexico and beyond (often near brackish water)
What is a Molly?
The molly (Poecilia sphenops) is a hardy, sociable livebearer that has been a mainstay of the freshwater hobby for decades. It reaches around 10 cm (4 in) — noticeably bigger than its livebearer relatives the guppy and platy — and comes in a remarkable range of colour forms and fin types: jet-black, silver, dalmatian-spotted, gold, and the tall-dorsal-finned sailfin variants. That variety, combined with its tolerant temperament and practical algae-grazing habit, makes the molly one of the most genuinely useful fish for a community setup.
Unlike many popular aquarium fish, the molly has a natural affinity for hard, alkaline water, and in the wild it frequents brackish coastal lagoons and estuaries. That chemistry preference is something every keeper should know upfront — it shapes every decision about tank-mates, water target and long-term health.
Where do Mollies come from?
Wild mollies originate across Central America, with the core range running from coastal Mexico south through Guatemala and Belize. They are strongly associated with warm, shallow, slow-moving water — marshes, backwaters, tidal rivers and coastal lagoons where the water is hard, alkaline and frequently brackish. Some populations live in almost full saltwater conditions.
This coastal-brackish history explains the molly’s unusual chemistry preferences among freshwater fish: they do well in high-hardness, high-pH tanks and are notoriously intolerant of the soft, acidic water that tetras and rasboras prefer. The many captive-bred colour forms available today are far removed from the drab wild type, but their water-chemistry needs remain unchanged.
What size tank does a Molly need?
The minimum is 75 litres (20 gallons), and that figure reflects the molly’s size rather than just a comfortable safety margin. A 10 cm (4 in) adult is a meaningful fish that generates proportional waste and needs genuine swimming room. Sailfin males, which can push past 12 cm (5 in), benefit from even more space.
Keep the tank longer than it is tall — mollies are active, open-water swimmers that cruise the middle zone and appreciate horizontal distance. Good filtration is important because the molly’s algae-grazing habit does not excuse a stingy nitrogen cycle; they produce waste like any other fish of their size. Plant the tank moderately with hardy, alkaline-tolerant species (java fern, vallisneria, hornwort) and leave clear swimming lanes through the centre. A lid is wise — mollies occasionally jump.
What water parameters do Mollies need?
- Temperature: 23–28 °C (73–82 °F)
- pH: 7.0–8.2 — the molly is one of the few popular fish that genuinely thrives at the higher end of this range.
- Hardness: 10–25 dGH — hard water is not just tolerated, it is preferred.
These numbers are wider than many guides suggest because captive-bred mollies are reasonably adaptable within their preferred range. What matters most is consistency: mollies stressed by soft or acidic water become susceptible to “molly disease” (shimmying, clamped fins), and that condition almost always traces back to water chemistry rather than a pathogen. If your tap water is naturally hard and alkaline, mollies will reward you. If your water is soft, consider pairing it with a livebearer buffer or choosing a different species.
What do Mollies eat?
Mollies are omnivores with a pronounced herbivorous lean. They graze algae continuously throughout the day — a useful trait in any planted or rock-scaped tank — and need a diet that reflects this. A quality omnivore or livebearer flake or pellet should form the base of their diet, supplemented regularly with blanched vegetables (zucchini, spinach, cucumber) and spirulina-based foods.
They also readily eat small live and frozen foods such as daphnia, brine shrimp and bloodworms, which help with condition and breeding. Feed small amounts two to three times daily and avoid overfeeding: uneaten plant matter rots quickly and drives ammonia spikes in a tank with the soft bioload mollies produce. A varied diet that keeps the herbivore side covered produces the best colour and the healthiest fish.
Are Mollies aggressive — and what fish can live with them?
Mollies are peaceful community fish in most circumstances. Males do exhibit mild competition with each other, particularly in the presence of females — chasing and fin-flaring are normal rather than injurious — but genuine aggression is rare. Keeping at least one male per two or three females (a minimum group of three) reduces the social pressure on any individual female.
The molly’s primary compatibility constraint is water chemistry, not temperament. Fish that need soft, acidic water (cardinal tetras, discus, most wild-caught South Americans) are a poor match not because of fighting but because their chemistry requirements are irreconcilable. Ideal tank-mates share the molly’s preference for hard, alkaline water: platies, guppies, swordtails, southern platyfish, most rainbowfish, livebearers, and robust community fish like corydoras or bristlenose pleco that tolerate the higher pH.
For a detailed, filterable list of compatible species, see Molly tank mates.
How do you tell male and female Mollies apart?
Sexing adult mollies is straightforward. Males are slimmer and smaller-bodied, with a gonopodium — a pointed, rod-shaped modification of the anal fin used for internal fertilisation — in place of the fan-shaped anal fin females carry. In sailfin males, the dorsal fin is dramatically tall, sometimes as wide as the fish is long, which is absent in females of the same variety.
Females are larger and noticeably fuller-bodied, particularly when carrying young — a gravid (pregnant) female develops a dark gravid spot near the rear of the abdomen that darkens as birth approaches. Size alone is a reasonable guide in an established group: the biggest, roundest fish in the tank is almost always female.
How do Mollies breed?
Mollies are livebearers: fertilisation is internal and the female gives birth to free-swimming fry rather than eggs. Breeding in a mixed group is essentially automatic — if you keep males and females together in acceptable water, fry will appear. A healthy female can produce 20–100 fry every 4–6 weeks, with gestation running approximately 60 days.
Fry are born large enough to eat crushed flake from day one and are not targeted heavily by adults in a well-planted tank with some cover. If you want to raise a larger proportion, move the heavily gravid female to a separate tank or breeder box just before birth and return her afterward. There is no parental care once birth is complete. The breeding difficulty is rated Easy — the main challenge is managing population rather than triggering spawning.
What are common Molly diseases?
Molly disease (shimmying) is the species-specific concern: affected fish shimmy or rock in place, clamp their fins, and lose energy. It is almost never a true infectious disease — it is a stress response to poor water chemistry, specifically water that is too soft, too acidic or too cold. Fix the water parameters first.
Ich (white-spot) presents as small white granules over the body and fins and is the most common disease across all tropical fish; it spreads quickly in a stressed or chilled tank. Fin rot — fraying, receding fins with a white or red border — is a bacterial condition that follows water-quality neglect. Velvet (a fine golden-dust sheen) is occasionally seen, particularly in new imports or post-shipping stress. Internal parasites can occur in wild-caught or poorly-sourced stock and show as hollow belly and wasting despite eating.
Prevention follows the same logic for all of these: stable, hard, warm water; a cycled tank; weekly partial water changes; and quarantining new fish for at least two weeks before introducing them to the main tank.
Health note: disease diagnosis and medication choices are outside the scope of a care profile. If fish are showing symptoms, confirm the diagnosis against a reputable veterinary or aquatic-health source before treating.
How long do Mollies live?
With consistent care in appropriate water, mollies typically live 3–5 years. Water quality is the dominant variable — fish kept in soft or acidic conditions age faster and succumb to chronic stress diseases earlier. Fish purchased from reputable breeders who have maintained them in hard water from birth tend to be hardier than those that have passed through multiple chemistry changes in the distribution chain. Provide the right chemistry from the start and mollies are rewarding, long-lived community residents.
Frequently asked questions
Do mollies need salt or brackish water?
Not necessarily, but they do want hard, alkaline water and tolerate brackish conditions well. Soft, acidic tanks suit them poorly, which is why they pair better with platies and guppies than with soft-water tetras.
How big do mollies get?
Common mollies reach about 10 cm, and sailfin types a bit more — larger than guppies or platies, so give them a 75 L+ tank with swimming room.
What you need to keep a molly
The baseline is a heated, filtered 75 L+ tank: a reliable heater to hold 23–28 °C (73–82 °F), a gentle filter that won't batter a molly in the current, and a tight-fitting lid. Cycle the tank fully before adding any fish.
As an Amazon Associate we earn from qualifying purchases — buying through these links costs you nothing extra.
