Molly Fish Tank Size: How Many Gallons Do Mollies Need?
Published: 2026-11-03 · FishyKart Blog
The right molly fish tank size depends on the type of molly. Short-finned mollies (black, Dalmatian, balloon and most colour forms) need a tank with a base of about 36 x 12 inches (90 x 30 cm), roughly 21 gallons (81 litres). Sailfin mollies grow larger and need about 23 gallons (87 litres), and the giant sailfin about 27 gallons (104 litres). These are the minimums from SeriouslyFish; because mollies breed quickly, a bigger tank is easier to keep stable.
Photo: Corpse89 - Wikimedia Commons, CC BY-SA 3.0
Molly Tank Size by Type
| Molly type | Max size (standard length) | Minimum base | Minimum volume |
|---|---|---|---|
| Short-finned molly (Poecilia sphenops): black, Dalmatian, balloon and other bred forms | 3.1 in (8 cm) | 36 x 12 in (90 x 30 cm) | about 21 gal (81 L) |
| Sailfin molly (Poecilia latipinna) | 5 in (12.5 cm) | 30 x 15 in (76 x 38 cm) | about 23 gal (87 L) |
| Giant sailfin molly (Poecilia velifera) | 6 in (15 cm) | 36 x 15 in (91 x 38 cm) | about 27 gal (104 L) |
Source: SeriouslyFish species profiles for Poecilia sphenops, P. latipinna and P. velifera. FishBase gives 7.5 cm standard length for P. sphenops.
Most mollies in shops are hybrids of these species, so the label is not always exact. A practical rule: if the male has a tall, sail-like dorsal fin, plan for the sailfin size.
How Many Mollies per Tank
The numbers below are our practical stocking guide for adult mollies with good filtration and weekly water changes. Keep more females than males: SeriouslyFish advises several females per male for short-finned mollies and at least two per male for sailfins.
| Tank size | Short-finned mollies | Sailfin mollies |
|---|---|---|
| 10 gallons (38 L) | Not recommended; fry or quarantine only | Not recommended |
| 20 gallons long (75 L) | About 4 (1 male, 3 females) | Not recommended |
| 29-30 gallons (110-115 L) | About 6 | 3-4 |
| 40 gallons breeder (150 L) | 8-10 | 5-6 |
| 55 gallons (210 L) | 10-12 plus tank mates | 6-8 plus tank mates |
Leave space for fry. FishBase records broods of 20 to 150 young for P. sphenops, so a tank that looks half empty today can be full in a few months. Plan a second small tank or a rehoming plan for extra young.
Why Tank Length Matters More Than Gallons
Mollies are active mid-water swimmers. A long tank gives them swimming room and lets females get away from males, which is why the minimum is given as a base size. A 20 gallon long (30 x 12 in) is better than a 20 gallon high, and a 29 gallon is a good all-round choice for a molly group. Male sailfins need extra length to show their dorsal fins; SeriouslyFish notes that hard, alkaline water and good water quality are needed for full fin development.
Water Settings for a Molly Tank
| Parameter | Short-finned molly | Sailfin molly |
|---|---|---|
| Temperature | 70-82°F (21-28°C) | 70-79°F (21-26°C) |
| pH | 7.0-8.5 | 7.0-8.5 |
| Hardness | 15-30 dGH | 15-35 dGH |
A target of about 76°F (24-25°C) suits both. Salt is not required: SeriouslyFish notes that the calcium and magnesium in hard water matter more than salt. If your tap water is soft, the molly fish temperature and water guide explains how to raise hardness and pH safely.
Setting Up the Tank
- Filter: rated for at least the tank volume. Mollies eat a lot and produce plenty of waste.
- Heater: needed if the room drops below about 72°F (22°C).
- Plants: hardy plants such as hornwort and Java fern give females and fry cover. Mollies also graze on soft algae.
- Lid: mollies can jump, especially when males chase females.
- Water changes: 20-30% weekly.
For compatible fish that fit the same tank, see the molly fish tank mates guide. For how long a well-kept molly can live, see molly fish lifespan.
Mollies at FishyKart
FishyKart sells black, white, chocolate, balloon and moon tail mollies, which are short-finned forms that fit the 21 gallon guideline above.
Browse Mollies & Platies → Molly Care Guide →