Species · Livebearers
Molly
Poecilia sphenops
A big, personable, hardy livebearer — but bigger and messier than guppies or platies, and the most firmly hard-water fish of the group. Give it space, hard water, and plan for fry.
Difficulty: BeginnerTropical community tank✓ Fact-checked care guide
Quick facts
| Adult size | ~8–10 cm / 3–4 in (sailfin types larger) |
|---|---|
| Minimum tank | ~75 L / 20 US gal — SeriouslyFish recommends a 90 × 30 cm base |
| Temperature | 24–27 °C / 75–81 °F (within the species range of 21–28 °C / 70–82 °F) |
| pH | 7.0–8.5 (alkaline) |
| Hardness | 15–30 dGH — a firmly hard-water fish |
| Filtration & flow | Cycled, robust filtration — mollies are messier than smaller livebearers |
| Diet | Omnivore with a strong plant/algae component — quality flake/pellet plus vegetables and small live/frozen |
| Social | Social — keep a small group; with mixed sexes keep 2–3 females per male |
| Temperament | Peaceful but bold and active |
| Lifespan | 3–5 years |
Overview
The molly (Poecilia sphenops, including the classic black molly) is a big, characterful livebearer with real personality. It is hardy and rewarding — but people underestimate how large, active and messy it is, and how firmly it wants hard, alkaline water.
It is a livebearer (breeds readily) and it interbreeds with other Poecilia, so keep just one molly species per tank. Most stock is farmed; choose healthy, active fish.
Water & tank
Mollies are the hardest-water fish of the common livebearers — pH 7.0–8.5 and 15–30 dGH suit them, and they do poorly in soft, acidic water. Some keepers add aquarium mineral salts; true brackish water is not required for the common molly but it appreciates mineral-rich water.
They are bigger and more active than guppies or platies — give them at least ~75 L / 20 US gal (SeriouslyFish recommends a 90 × 30 cm base) with strong, cycled filtration for their heavier waste load.
Feeding
Omnivores that graze a lot of plant matter and algae in the wild — offer a quality flake/pellet plus blanched vegetables (and some small live/frozen foods). The vegetable component matters more than for other livebearers.
Feed modestly once or twice a day; their size makes overfeeding a fast water-quality problem.
Behaviour & tankmates
Generally peaceful but bold and busy — best with other robust, hard-water community fish of similar size. They can nip or out-compete very timid or tiny tankmates.
Keep 2–3 females per male, and only one molly species to avoid hybridising.
What to expect
A confident, interactive fish that recognises feeding time. Expect fry from a mixed group; the bigger bioload means water changes matter even more here.
Myths, kindly busted
“Mollies are small starter fish like guppies.”
They reach 8–10 cm+, are active and messy, and need ~75 L+ with strong filtration — a bigger commitment than a guppy or platy.
“Mollies need saltwater / brackish water.”
The common molly does not require brackish water — but it does need hard, alkaline, mineral-rich water to thrive.
When to act
We triage, not diagnose — but these are worth a closer look (starting with a water test):
- The "shimmies" (rocking in place without moving forward) — often a sign of poor or too-soft water; test and correct water quality/hardness.
- Clamped fins, gasping or lethargy — test the water first; mollies are sensitive to a dirty or under-filtered tank.
Sources & further reading
- SeriouslyFish — Poecilia sphenops — Adult ~80 mm SL; recommended 90 × 30 cm base (large tank); temperature 21–28 °C / 70–82 °F; pH 7.0–8.5; hardness 15–30 dGH (hard water); peaceful; livebearer; interbreeds with other Poecilia (keep one species).
Written & fact-checked by The Finfern Team against the sources above — kind, general guidance, not a diagnosis. If something looks wrong, test your water and, if it persists, contact a fish-savvy vet.
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