Keeping mollies is a favorite choice among freshwater aquarium enthusiasts.
They are hardy, colorful, and easy to care for, making them perfect for beginners and experienced aquarists alike.
However, one of the most common questions people ask is: How many mollies can you safely keep in a 20-gallon tank?
In this guide, we’ll answer that question in detail and also cover the factors that affect stocking, care tips, and best practices to ensure your mollies thrive.
How Many Mollies Can Fit in a 20-Gallon Tank?
On average, you can comfortably keep 4 to 5 mollies in a 20-gallon tank. This is based on the general rule of one inch of fish per gallon of water and the fact that mollies grow to about 3–4.5 inches long as adults.
Here’s a quick breakdown:
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4–5 mollies = ideal stocking
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6 mollies = possible but requires excellent filtration and regular water changes
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More than 6 = overcrowding risk, leading to stress and poor water quality
Why Tank Size Matters for Mollies
Mollies are active swimmers, and even though they are small, they need enough open space to explore and move around freely. Overcrowding a tank can cause stress, aggression, and disease. A 20-gallon tank provides a manageable balance between space and maintenance.
Factors That Affect Stocking in a 20-Gallon Tank
1. Size of the Mollies
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Balloon mollies are smaller and require less space.
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Sailfin mollies grow larger and need more room.
2. Gender Ratio
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Male mollies can be territorial.
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The best ratio is 1 male for every 2–3 females to reduce aggression and prevent stress.
3. Filtration System
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A powerful filter helps maintain water quality.
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Overcrowding without good filtration will quickly lead to ammonia spikes.
4. Tank Setup and Decorations
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Live plants, rocks, and driftwood create hiding spaces.
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Proper aquascaping helps mollies feel secure.
5. Tankmates
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Mollies are peaceful but active.
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Good companions include guppies, platies, and swordtails. Avoid aggressive fish like cichlids.
Best Practices for Keeping Mollies in a 20-Gallon Tank
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Maintain Water Quality – Keep temperature between 75–80°F, pH between 7.5–8.5, and perform weekly 20–25% water changes.
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Provide a Balanced Diet – Feed high-quality flakes, pellets, and occasional vegetables like zucchini or spinach.
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Avoid Overcrowding – Stick to the recommended number of mollies to prevent stress.
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Use a Heater and Filter – Mollies are tropical fish and need consistent warm water with good filtration.
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Add Plants and Hiding Spots – Live plants not only help water quality but also reduce stress in fish.
Common Mistakes to Avoid
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Adding too many mollies – leads to aggression and poor health.
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Neglecting gender ratios – too many males will fight.
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Skipping water changes – mollies are hardy, but bad water conditions harm them.
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Using a small filter – always match your filter size to your tank and bioload.
Conclusion
In a 20-gallon tank, the sweet spot is 4 to 5 mollies.
This allows them to swim freely, stay healthy, and live stress-free.
Always remember that filtration, maintenance, and proper gender ratios play a big role in keeping mollies happy.
By following these guidelines, your 20-gallon aquarium will become a lively, colorful home for your mollies.
FAQs
1. Can I keep 10 mollies in a 20-gallon tank?
No, that would overcrowd the tank and harm water quality. Stick to 4–5 mollies.
2. Can mollies live with guppies in a 20-gallon tank?
Yes, mollies and guppies make great tankmates as they share similar water requirements.
3. Do mollies need a heater in a 20-gallon tank?
Yes, mollies are tropical fish and need a heater to maintain 75–80°F.
4. How often should I clean a 20-gallon molly tank?
Perform partial water changes once a week and clean the filter monthly.
5. Can I breed mollies in a 20-gallon tank?
Yes, but remember mollies reproduce quickly. Overbreeding can overcrowd the tank.