What Size Tank Do Mollies Need?
Adult standard length 3.1 inches. The published tank is 36 × 12 × 12 inches, about 21 US gallons.
Full breakdown: how many fish per gallon. Full breakdown: how many fish in a 20 gallon tank.
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The figures
| Figure | Value | Source |
|---|---|---|
| Adult standard length | 3.1 in | Seriously Fish |
| Minimum aquarium | 36 × 12 × 12 in | Seriously Fish |
| Approximate volume | 21 US gallons | Seriously Fish |
| Group / social | No group count is given on the profile figures used here | Seriously Fish |
| Filter rating | 325-405 liters per hour | Seriously Fish |
| Filter rating in gallons per hour | 85.87-107.00 gph | Calculated from the published liters per hour ÷ 3.785 (Seriously Fish; 1 US gallon = 3.785 liters) |
| Temperature | 70-82°F | Seriously Fish |
| Heater on the profile | 75-100 watts, assuming a room around 68°F | Seriously Fish |
| Aqueon band that contains about 21 gallons | 100 watts for 15-40 gallons | Aqueon |
| Aqueon 50 watt band | 5-20 gallons (21 is above 20) | Aqueon |
Full breakdown: what size filter for a 20 gallon tank.
How to size it
The floor matches the zebra danio minimum, and the molly profile adds a 12 inch height: 36 × 12 × 12 inches, about 21 US gallons (Seriously Fish). The fish is larger than a danio. Adult standard length is 3.1 inches, against the danio's 2.0 inches. No group count is printed here, while the danio profile asks for at least 8-10. Do not copy the danio headcount onto mollies.
Four times about 21 gallons is 84 gallons per hour, and five times is 105. The converted 85.87-107.00 gph follows that rule. Flow is not marked gentle.
Heat is where mollies line up with the volume chart and danios do not. The profile lists 70-82°F and 75-100 watts, assuming a room around 68°F. Aqueon lists 100 watts for 15-40 gallons. About 21 is inside that band. It is above 20, so it is outside the 50 watt band, and it is below 30, so it is outside the 150 watt band. 100 watts is the shared figure: the top of the profile's 75-100 watt span and the Aqueon rating for this volume.
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Accessory: a 100 watt heater, the top of the profile's 75-100 watt span and the Aqueon band for about 21 gallons. Shop a 100 watt aquarium heater on Amazon
Related pages
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FAQ
What size tank do mollies need?
Seriously Fish lists 36 × 12 × 12 inches, about 21 US gallons, for Poecilia sphenops. Adult standard length is 3.1 inches. The profile figures used here do not include a group count.
What filter rating is listed for mollies?
Seriously Fish lists 325-405 liters per hour for Poecilia sphenops on the 36 × 12 × 12 inch tank. Dividing by 3.785 gives 85.87-107.00 gallons per hour. Four times about 21 gallons is 84 gallons per hour, and five times is 105.
What heater wattage matches about 21 gallons for mollies?
Seriously Fish lists 70-82°F and a 75-100 watt heater, assuming a room around 68°F. Aqueon lists 100 watts for 15-40 gallons, which includes about 21 gallons, and 50 watts for 5-20 gallons, which does not.
Sources
- Seriously Fish, Poecilia sphenops. https://www.seriouslyfish.com/species/poecilia-sphenops/. Accessed Oct 2, 2026.
- Aqueon, Heater Wattage Guidelines. https://www.aqueon.com/-/media/project/oneweb/aqueon/files/us/educational/aq_heater_wattage_chart.pdf. Accessed Oct 2, 2026.