How many mollies can you keep in a 5 gallon?

Mollies are a popular freshwater aquarium fish that are often kept by beginner and experienced fishkeepers alike. Their peaceful nature, easy care requirements and wide variety of color variations make them an ideal community fish. However, there is some debate among aquarists about the ideal number of mollies that can be housed together in a small enclosed space like a 5 gallon tank. In this article, we’ll take a look at the key factors to consider when stocking mollies and provide recommendations for how many can comfortably be kept in a 5 gallon aquarium.

The Typical Size of Molly Fish

Mollies are relatively small fish, typically growing to between 2-4 inches in length when fully grown. The most commonly kept mollies are sailfin mollies and black mollies. Within these groups there are a number of color varieties including gold dust, black, dalmatian and more. Regardless of the specific color type, the average mature molly size is around 3 inches.

This small size makes them suitable tank mates for similarly sized community fish. It also means that a number of them can be kept in a relatively small tank volume compared to larger fish species. However, each fish still requires adequate space to swim, feed and exhibit natural behaviors. Mollies are active swimmers that utilize all areas of the tank.

General Stocking Guidelines for Community Aquariums

When stocking any community aquarium, there are some general guidelines to follow regarding the number and size of fish:

  • Allow 1 inch of fully grown adult fish per 1 gallon of water
  • Stock less for active swimmers like mollies
  • Choose tank mates of similar size and temperament
  • Provide adequate filtration for the tank volume and bioload
  • Allow open areas for swimming and heavily planted areas for hiding/retreating

Following these basic rules helps prevent aggressive behavior and territorial disputes by reducing competition for resources like food, shelter and swimming space.

Factors to Consider for a 5 Gallon Molly Tank

When looking specifically at keeping mollies in a 5 gallon tank, there are a few additional considerations to take into account:

Tank Dimensions

The footprint and dimensions of a 5 gallon tank are also important. Most standard 5g tanks have a square shaped base measuring around 16 inches x 8 inches. They are shallow, often less than 1 foot deep. The small surface area limits swimming room for active fish.

Water Parameters

Water quality declines more rapidly in smaller volumes. A 5g requires extra filtration and frequent testing to maintain clean, stable water parameters ideal for mollies. They prefer slightly brackish water with some salt added. Excellent filtration and frequent partial water changes are a must.

Tank Decor

A 5g tank should be decorated intentionally to create sight barriers and break up lines of sight between fish. Live plants, rocks and driftwood optimize the usable space. Open areas should be available for swimming along with planted zones for retreating.

Tank Mates

Any tankmates must be equally small and peaceful to coexist with mollies in a 5 gallon environment. Appropriate options include: neon tetras, guppies, endler’s livebearers, pygmy corydoras, snails like nerites, and shrimp like ghost or cherry shrimp.

Recommended Molly Stocking for a 5 Gallon Aquarium

Taking all the above factors into account, here are some general guidelines for safely and responsibly stocking mollies in a 5 gallon tank:

  • 3-5 mollies max
  • If including tank mates, reduce number of mollies to 2-3
  • Tank mates should be nano fish/inverts under 2 inches in size
  • 1 betta can be combined with shrimp and snails
  • Increase to 20-30% water changes weekly
  • Heavily plant the tank and use decor to create barriers
  • Supplement diet with vegetables to reduce waste

This conservative stocking helps prevent territorial aggression and allows room for mollies to freely swim and exhibit natural schooling behaviors. Remember that mollies breed easily and populations can quickly get out of control in small tanks.

3 Mollies

A good starting point is 3 mollies in a 5 gallon tank. This allows room for them to establish some hierarchy and spacing between individuals. Reduce to 2 mollies if including a cleanup crew of shrimp or snails. The remaining space can be used to create sight barriers with live plants and decor.

4 Mollies

Some experienced aquarists have successfully kept 4 mollies in a heavily planted 5 gallon tank. This is nearing the maximum capacity and should only be attempted if you are diligent with tank maintenance and water testing. The extra bioload demands more frequent water changes.

5 Mollies

It’s generally not recommended to keep 5 mollies in a 5 gallon tank. The overcrowding increases aggression and stress as they compete for space and resources. Disease outbreaks, failed water quality and premature death become much more likely.

Providing the Best Care for Mollies

Caring for mollies in small tanks can be done successfully but requires an experienced aquarist willing to put in extra effort. Here are some tips:

  • Test water 2-3x per week and change at least 30% twice a week
  • Feed a high quality omnivore diet with added spirulina and vegetables
  • Maintain slightly brackish water with aquarium salt
  • Use a sponge filter or air powered filtration rated for 10+ gallon tanks
  • Perform weekly tank grooming to remove waste
  • Quarantine new mollies for 2-4 weeks before introducing to the main tank

With close monitoring and care, a small group of mollies can thrive in a 5 gallon aquarium. Success requires a commitment to frequent maintenance and creating an environment tailored to their needs.


When stocking mollies in a 5 gallon tank, the recommended limit is 3-4 individuals depending on if other tank mates are included. Several factors must be optimized to support good health in such a small space including tank shape, filtration, water quality, diet, decor and strict population control. While a 5 gallon tank can humanely house a trio of mollies, larger tanks of 10-20 gallons are easier to maintain stable conditions in and are highly recommended.

Leave a Comment