TankStocking

How many Molly (Common / Sailfin) in a 20 gallon tank?

A 20-gallon (76 L) tank comfortably holds about 3–4 Molly (Common / Sailfin) as a single-species display — a conservative range that leaves room for tankmates and a margin for error.

By the TankStocking team · methodology-driven, sourced from aquarium care references Data v2026.06 · last reviewed How we judge this →

Molly (Common / Sailfin) key facts

Key facts — Molly (Common / Sailfin) (Poecilia sphenops / Poecilia latipinna)
Adult size12 cm
Minimum tank20 US gal
Minimum group3+ (pair/group)
TemperamentPeaceful
Temperature range24–28°C
pH range7.5–8.5
BioloadHigh
Swim levelAll levels
Beginner-friendlyYes
Sources & confidence (1 species)

This backs the Molly (Common / Sailfin) size, tank and group figures used here. Each figure is read from the TankStocking species database (v2026.06); below is the care reference behind it and how confident we are in that data. Confidence reflects the source quality, not whether any pairing is safe. Full source list and the welfare model are on the methodology page.

  • Molly (Common / Sailfin) Poecilia sphenops / Poecilia latipinna — Aquarium Co-Op molly care guide / FishBase Poecilia latipinna high confidence

TankStocking sizes this from a per-species footprint — the territory and waste a single Molly (Common / Sailfin) needs in a comfortable display tank — rather than the unreliable "inch per gallon" rule. On that basis a 20-gallon tank comfortably suits about 3–4 Molly (Common / Sailfin). Keep a minimum of 3 — Molly (Common / Sailfin) is a social species that suffers in smaller groups. This is a comfortable display range, not a hard ceiling: stocking lighter keeps water quality stable and leaves headroom for tankmates and growth.

They get along with most peaceful fish but are larger and messier than guppies, so they need more space and stronger filtration. The engine clears the usual community crowd; plan for breeding and for their preference for harder water (some keepers add a little salt, which limits invertebrate tankmates).

Want a community rather than a single-species tank? The Molly (Common / Sailfin) tank mates guide lists the species the engine clears with Molly (Common / Sailfin).

4× Molly (Common / Sailfin)

✓ Good starter plan

4× Molly (Common / Sailfin)

About 3–4 Molly (Common / Sailfin) is a comfortable display level for 20 gallons — fewer is always safer, and leaves headroom for tankmates.

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      Common questions

      How many Molly (Common / Sailfin) can I keep in a 20-gallon tank?

      A 20-gallon (76 L) tank comfortably holds about 3–4 Molly (Common / Sailfin) as a single-species display — a conservative range that leaves room for tankmates and a margin for error.

      What size tank do Molly (Common / Sailfin) need?

      Molly (Common / Sailfin) need at least 20 US gallons, kept in a group of 3 or more. Bigger is better — more water means more stable parameters and more room to stock tankmates.

      More on stocking Molly (Common / Sailfin)

      Related guides on TankStocking — each scored by the same welfare engine as the planner.

      Molly (Common / Sailfin) tankmates & community

      Molly (Common / Sailfin) in other tank sizes

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      Stocking numbers are planning estimates — filtration, planting, and maintenance all shift the real ceiling. Cycle the tank and stock gradually. How this estimate works →