Can I put 4 mollies in a 10-gallon tank?

The short answer is yes, you can keep 4 mollies in a 10-gallon tank. Mollies are a popular freshwater fish that do well in community aquariums. With proper tank setup and care, 4 mollies will thrive in a 10-gallon tank.

Optimal Tank Size for Mollies

Mollies are active fish that grow to 2-3 inches in length. They enjoy swimming horizontally across the tank. The general rule is to provide at least 10 gallons of water for the first molly, and 2-3 gallons for each additional molly in the tank.

A 10-gallon tank is an acceptable minimum size for a group of 4 mollies. However, a larger tank is always better and provides more ample space for them to freely swim. A 20-gallon long tank is ideal for a small group of 4-6 mollies.

Tank Setup for 4 Mollies

When keeping multiple mollies, the aquarium needs to be set up properly to support the bioload. Here are some tips for setting up a 10-gallon molly tank:

  • Substrate – Sand or fine gravel substrate
  • Filter – Hang-on-back filter rated for a 20-gallon tank
  • Heater – Preset heater to maintain 75-80°F
  • Plants – Floating, silk or live plants for hiding spots
  • Decor – Rocks, driftwood, cave decor for hiding
  • Lighting – Standard aquarium lighting

A powerful filter is needed to handle the waste produced by 4 mollies in a 10-gallon tank. You want at least double the filtration capacity for the size tank. Use a filter rated for 20-gallons or more.

Live plants can help absorb some of the nitrates but are not mandatory. Be sure to include plenty of plants and decor for places to hide and explore.

Ideal Water Parameters for Mollies

Mollies thrive in relatively hard, alkaline water. The ideal water parameters are:

  • Temperature – 75-80°F
  • pH – 7.0-8.0
  • Hardness – 10-25 dGH

Regular partial water changes will be required to maintain good water quality. Change out 25% of the water weekly. Use a gravel vacuum to remove waste and do not disrupt the nitrogen cycle.

Compatible Tank Mates for Mollies

Mollies are peaceful community fish that do well with other similarly-sized fish. Some compatible molly tank mates include:

  • Swordtails
  • Platies
  • Guppies
  • Corydoras catfish
  • Tetras
  • Plecos

Avoid aggressive fish like tiger barbs that may nip at mollies. Also do not house mollies with fish that prefer different water parameters.

Feeding 4 Mollies in a 10-Gallon Tank

Mollies are omnivores that need both protein-rich foods and plant-based foods. Offer a varied diet such as:

  • Flake food
  • Freeze-dried bloodworms
  • Brine shrimp
  • Vegetable matter
  • Algae wafers

Feed them small amounts 1-2 times per day. Remove any uneaten food to avoid polluting the tank. Overfeeding can foul the water quickly in a small tank.

Molly Tank Maintenance

Perform regular molly tank maintenance to keep the water clean and fish healthy in a 10-gallon aquarium. Follow this routine:

  • Weekly 25% water changes
  • Test water parameters weekly
  • Clean filter monthly
  • Wipe down glass algae as needed
  • Trim plants as needed

Closely monitor ammonia, nitrite, and nitrate levels. Mollies have a high bioload and the small tank volume makes water quality management critical.

Breeding Mollies in a 10-Gallon Tank

Mollies will readily breed in a 10-gallon tank. The breeding process is relatively straightforward:

  • House one male and one female molly together
  • Provide good water quality and a healthy diet
  • The female will give birth to 20-40 fry
  • Leave fry in the tank or move them to a separate rearing tank

However, breeding mollies in a 10-gallon tank can quickly lead to overcrowding. It is recommended to move the pregnant female to a breeding tank so you can control the fry population. The 10-gallon tank simply won’t have room for an unexpected molly explosion.

The Pros of Keeping 4 Mollies in a 10-Gallon Tank

Here are some of the benefits of housing 4 mollies in a 10-gallon tank:

  • Active schooling fish for a small tank
  • Relatively hardy fish, good for beginners
  • Peaceful community fish
  • Fun to watch them interact and swim together
  • Brings color and activity to a small tank

A group of 4 mollies creates an active community in a 10-gallon aquarium. Their schooling behavior is entertaining to observe in a compact space.

The Cons of Keeping 4 Mollies in a 10-Gallon Tank

While doable, there are also some downsides to keeping 4 mollies in a 10-gallon tank:

  • Increased filtration requirements
  • Frequent water changes needed
  • Possibility of stunting molly growth
  • Risk of breeding leading to overstocking
  • Less swimming room than ideal

The main difficulties involve maintaining good water quality. Their bioload will quickly deteriorate water conditions without diligent tank maintenance. You also run the risk of stunting their growth unless you upgrade tanks later on.


In summary, it is possible to house 4 mollies in a 10-gallon tank. However, a larger tank is preferable to give them adequate swimming space. If keeping them in a 10-gallon aquarium, be prepared to stay on top of tank maintenance and filtration requirements. Perform frequent water changes, feed a proper diet, and test the water regularly. With good husbandry, 4 mollies can thrive in a small community tank.

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