How Many Molly Fish In A 30-Gallon Tank? (With Examples)

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Molly fish are one of the most common freshwater aquarium fish, and for two main reasons: they are incredibly beautiful and friendly.

If you are just starting your 30-gallon tank and wonder how many mollies you can add to it – you have come to the right place.

In this article, I will discuss the right amount of molly fish suitable for that tank size, and suggest compatible fish species and quantities for mixing with mollies.

Let’s get started.

How Many Molly Fish Can I Keep In A 30-Gallon Tank?

For a 30-gallon tank, you can safely accommodate six to seven adult molly fish.

These fish are known for their social behavior and thrive in groups, but also need ample space to swim freely.

This is where the 1-inch per gallon rule is generally used, accounting for their full-grown size.

How To Calculate The Number Of Molly Fish Suitable For A 30-Gallon Tank

You can calculate the number of molly fish in a tank of 30 gallons by following the one-inch-per-gallon rule:

  • Determine the adult size of molly fish, typically around 4-5 inches.
  • Follow the one-inch-per-gallon rule; for mollies, use a 5-gallon per fish ratio.
  • Divide the total tank volume (30 gallons) by the volume needed per fish (5 gallons).
  • The result, 6-7, is the suitable number of molly fish for your tank.

Why Is A 30-Gallon Tank Considered Ideal For Molly Fish?

A 30-gallon tank is considered ideal for molly fish because it provides them with sufficient space to swim and thrive, and it’s manageable for keeping the water parameters stable.

This size of tank aligns well with the physical needs and behavioral patterns of molly fish. Here’s why:

  • Space for Swimming: Molly fish are active swimmers and require ample space. A 30-gallon tank offers them enough room to swim freely, contributing to their well-being and reducing stress.
  • Water Quality: Larger tanks like a 30-gallon one help to dilute toxins, making it easier to maintain stable water conditions, a crucial aspect for the health of molly fish.
  • Social Behavior: Molly fish are sociable creatures that enjoy being in groups. A 30-gallon tank allows for an appropriate number (6-7) of molly fish to interact, forming a healthy community.
  • Breeding Space: If you intend to breed molly fish, a 30-gallon tank gives enough space for the young to grow and reduces the chance of predation by adults.
  • Decorations and Plants: Molly fish appreciate a well-decorated tank with plenty of plants. A 30-gallon tank provides adequate room for such decorations, creating a more enriching environment for molly fish.

Also Read: Molly Fish Tank Size

Maintenance Tips And Tricks For A 30-Gallon Tank With Molly Fish

Maintaining a 30-gallon tank with molly fish involves regular cleaning, monitoring water parameters, and creating an enriching environment.

These tasks ensure the well-being of your molly fish and keep the tank looking vibrant. Here are some key tips and tricks:

  • Regular Cleaning: A weekly 10-20% water change can help maintain optimal water conditions for molly fish. Always use dechlorinated water to avoid harming your fish.
  • Water Parameters: Monitor pH levels (7.5-8.5), hardness, and temperature (72-82°F) frequently. Mollies prefer slightly alkaline and hard water, and a warm temperature.
  • Proper Filtration: A good filtration system is essential to remove waste and toxins. Choose a filter rated for at least 30 gallons to best support your molly fish. My recommendation: Fluval C4 Power Filter (link to Amazon).
  • Balanced Diet: Feed mollies a mix of high-quality flake food, brine shrimp, and vegetable matter. Overfeeding can lead to water quality issues, so feed only what they can consume in 2-3 minutes.
  • Tank Mates: Select tank mates carefully. Compatible mates for molly fish include platies, guppies, and other peaceful species.
  • Plants and Decorations: Mollies enjoy well-planted tanks. Incorporate live plants, caves, and decorations for hiding and breeding spots.
  • Quarantine New Fish: Always quarantine new fish before adding them to the main tank to prevent the spread of diseases to your existing molly fish.

What Are The Consequences Of Adding Too Many Molly Fish?

Overpopulation of molly fish can result in a multitude of issues including, compromised water quality, escalated risk of diseases, and amplified stress among the fish.

Here are the details:

  • Stress Levels: Having too many molly fish in a limited space can induce stress, leading to impaired immunity and increased susceptibility to infections.
  • Water Quality: A densely populated tank can expedite the deterioration of water quality due to heightened waste production, leading to potential spikes in harmful substances like ammonia and nitrites.
  • Disease Spread: Disease transmission becomes more likely when molly fish are crammed in close quarters, posing a risk to the health of all fish in the tank.
  • Resource Scarcity: More fish implies more competition for resources like food, which may lead to malnutrition in some fish and overfeeding in others, each with their own associated health risks.
  • Aggression: Overpopulation could also result in territorial conflicts among molly fish, causing physical harm and additional stress.

Also Read: How Many Molly Fish In A 20-Gallon Tank?

Does The Gender Of Molly Fish Matter?

No, the molly fish’s gender doesn’t significantly impact their suitability for a 20-gallon tank. 

Both males and females exhibit similar sizes and behaviors, making both genders equally appropriate for this tank size.

The main considerations should be their overall health and how well they’ll coexist with the existing fish population.

Is Breeding Molly Fish Possible In A 30-Gallon Setup?

Yes, breeding molly fish is possible in a 30-gallon setup, providing enough space for the fry to grow and minimizing predation risks.

Here are some tips for successful molly fish breeding in a 30-gallon tank:

  • Separate Breeding Tank: Consider setting up a separate breeding tank within the 30-gallon setup to isolate pregnant females and protect the fry from adult mollies.
  • Ideal Water Conditions: Maintain stable water parameters, including a temperature around 78-82°F, slightly alkaline pH (7.5-8.5), and moderate hardness. These conditions mimic their natural habitat and promote successful breeding.
  • Feeding High-Quality Foods: Provide a varied diet with high-quality flake food, small live or frozen foods, and vegetable matter. A nutritious diet supports the health of the breeding mollies and ensures proper growth of the fry.
  • Plenty of Hiding Places: Incorporate dense plantings, floating plants, and artificial or natural hiding spots like caves or PVC pipes. These provide shelter for the fry, reducing predation risks and encouraging breeding behavior.
  • Separate Fry from Adults: Once the fry are born, transfer them to the breeding tank or a separate nursery setup to ensure their safety and allow them to grow without competition or aggression from adult mollies.

Also Read: How Many Molly Fish In A 40-Gallon Tank?

Can I Add More Molly Fish To A 30-Gallon Tank?

Yes, it is possible to introduce additional molly fish into a 30-gallon tank (more than 6-7). However, I highly encourage you to do that properly:

  • Gradual introductions: Introduce new molly fish slowly to allow them to acclimate and minimize stress.
  • Monitor water parameters: Regularly test and maintain appropriate levels of ammonia, nitrite, nitrate, pH, and temperature for the well-being of the molly fish. I personally use the API FRESHWATER MASTER TEST (link to Amazon).
  • Adequate filtration: Ensure your filtration system can handle the increased bioload and maintain optimal water quality.
  • Watch for aggression: Monitor the behavior of existing molly fish and provide additional hiding spots if territorial disputes arise.
  • Consider stocking limit: Keep in mind the adult size and the one-inch-per-gallon rule to avoid overstocking and maintain a healthy environment.

How Many Molly Fish Should Typically Be Kept Together?

To promote social interaction and maintain a harmonious community, it is often recommended to keep a group of 4-5 molly fish.

This number allows the fish to engage in their natural behaviors, such as schooling and mating, while preventing overcrowding and excessive competition for resources.

Keeping Molly Fish With Other Fish In A 30-Gallon Tank

Keeping only molly fish in 30 gallons can be a bit boring. Here is a table discussing other types of fish you can introduce:

Fish CombinationNumber of Fish (30-Gallon Tank)
Only Molly Fish6-7 mollies
Molly Fish + Guppies6 mollies + 12 guppies
Molly Fish + Neon Tetras6 mollies + 15-20 neon tetras
Molly Fish + Zebra Danios6 mollies + 10-12 zebra danios
Molly Fish + Platy Fish6 mollies + 10-12 platies
Molly Fish + Betta Fish6 mollies + 1 betta fish

1. Molly Fish And Guppies

In a 30-gallon tank, you can typically keep around six mollies and a dozen guppies.

Both species are peaceful and enjoy similar water conditions, making them compatible tank mates.

2. Molly Fish And Neon Tetras

You could house six mollies along with 15-20 neon tetras in a 30-gallon tank.

Neon tetras are small, peaceful fish that will generally stay out of the mollies’ way and they share similar water condition needs.

3. Molly Fish And Zebra Danios

In a 30-gallon tank, you can maintain about six mollies and 10-12 zebra danios.

Danios are fast, active swimmers that usually inhabit the upper part of the tank, minimizing potential conflict with mollies.

4. Molly Fish And Platy Fish

You could keep six mollies and about 10-12 platies in a 30-gallon tank. Platies are peaceful, hardy fish that get along well with mollies, and they both enjoy similar water conditions.

5. Molly Fish and Betta Fish

For a 30-gallon tank, it’s feasible to have six mollies and one betta fish.

Betta fish can sometimes be territorial, but in a sufficiently large and well-decorated tank, they can coexist with mollies. However, watch for any signs of aggression from the betta.


If you are in a rush, here is a brief overview of what I discussed earlier:

  • A 30-gallon tank can comfortably accommodate six to seven adult molly fish, considering their social behavior and need for swimming space.
  • The one-inch-per-gallon rule is commonly used to calculate the number of molly fish suitable for a 30-gallon tank, with a recommended 5-gallon per fish ratio for mollies.
  • A 30-gallon tank is ideal for molly fish due to its sufficient space for swimming, maintenance of water quality, promotion of social behavior, and providing breeding opportunities.
  • Maintaining a 30-gallon tank with molly fish requires regular cleaning, monitoring water parameters, and creating a well-decorated environment with suitable tank mates.
  • Overpopulation of molly fish in a tank can lead to compromised water quality, increased disease risks, heightened stress levels, resource scarcity, and potential aggression.