How Many Molly Fish In A 10-Gallon Tank? (Quick Answer)

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If you own a 10-gallon tank and wish to grow molly fish in it, you have come to the right place.

In this article, I will discuss how many mollies can be kept in 10 gallons, and whether it’s possible to mix other species with them in that tank size.

I will also show you why 10 gallons might be too small, and what steps you can take if you wish to use that tank size.

Let’s get started.

How Many Molly Fish Can You Keep In 10 Gallons?

To provide a suitable living space for molly fish, it is important to consider their size and swimming needs.

Adult molly fish can reach lengths of up to 4.5 inches, so they require enough room to move around and thrive.

In a 10-gallon aquarium, it is generally recommended to keep around 2 to 3 adult molly fish. 

This allows each fish to have sufficient space for swimming comfortably and engaging in natural behaviors.

Calculating The Number Of Molly Fish For A 10-Gallon Tank

Here is how you can calculate the number of molly fish in a tank of 10 gallons, following the one-inch-per-gallon rule:

  • The 1-inch-per-gallon rule is a guideline suggesting one inch of fish per gallon of water.
  • Molly fish grow to a maximum length of about 4.5 inches.
  • Based on the rule, divide the 10-gallon tank capacity by the adult size of molly (4.5 inches) to determine the suitable number of fish.
  • The calculation would be 10 (gallons) / 4.5 (inches) which equals roughly 2.2.
  • Since you can’t have 0.2 of a fish, round down to the nearest whole number.
  • Therefore, using the 1-inch-per-gallon rule, a 10-gallon tank could house 2 full-grown molly fish comfortably.

What Is The Recommended Tank Size For Molly Fish?

The recommended tank size for molly fish is at least 20-30 gallons for a small group. This allows them to have plenty of room to swim and engage in their natural behaviors.

Here’s why this recommendation is crucial:

  • Space Requirement: Molly fish can grow up to 4.5 inches long. More space means they can swim freely, reducing stress and contributing to their overall health.
  • Social Behavior: Molly fish are social creatures. A larger tank can accommodate a small group, allowing for social interaction and reducing the risk of territorial aggression.
  • Water Quality: Larger tanks help maintain stable water parameters. Molly fish require clean, well-filtered water, and larger tanks are easier to manage in terms of water quality.
  • Breeding Purpose: Molly fish are prolific breeders. If you’re planning on breeding, extra space is required to accommodate the offspring.
  • Decoration and Plants: Molly fish enjoy environments with hiding places and vegetation. A larger tank allows for adequate decoration without compromising swimming space.

Also Read: Molly Fish Tank Size

Why 10 Gallons May Not Be As Spacious As You Think

While a 10-gallon tank may seem substantial, it may not provide as much space for molly fish as one might think. 

Considering their size, social behavior, and environmental needs, a 10-gallon tank can quickly become cramped.

Here’s why a 10-gallon tank might not be as spacious:

  • Adult Size: Molly fish can grow up to 4.5 inches. Given their size, a 10-gallon tank could accommodate only about two full-grown mollies comfortably.
  • Group Behavior: Molly fish are social and thrive in small groups. A 10-gallon tank restricts the number of fish you can house, potentially limiting their social interactions.
  • Breeding Concerns: Molly fish are known to breed frequently. A 10-gallon tank may not provide sufficient space for the offspring, leading to overcrowding.
  • Decor and Plant Life: Mollies enjoy environments with hiding places and vegetation. Adding these elements to a 10-gallon tank could further reduce the available swimming space.
  • Water Quality Management: Smaller tanks can be more challenging to maintain in terms of stable water parameters. This could impact the health of your molly fish if not carefully managed.

Also Read: How Many Molly Fish In A 5-Gallon Tank?

Maintenance Tips And Tricks For A 10-Gallon Tank

Maintaining a 10-gallon tank for molly fish involves careful attention to water quality, temperature, feeding, and tank conditions.

Even with the size constraints, there are ways to ensure the health and happiness of your molly fish.

Consider the following:

  • Regular Water Changes: Weekly changes of about 25-50% of the water can help maintain water quality. Regular changes can help control waste buildup, benefiting your molly fish’s health.
  • Proper Filtration: A good filter is crucial in a 10-gallon tank. It keeps water clean and oxygenated, creating a healthier environment for molly fish. My recommendation: Fluval C4 Power Filter (link to Amazon).
  • Temperature Control: Molly fish prefer warmer waters, between 70-82°F (21-28°C). Consistent monitoring and a reliable heater can help maintain suitable temperatures. I personally got the Orlushy Submersible Aquarium Heater (link to Amazon).
  • Feeding Moderation: Overfeeding can cause water quality issues. Feed your molly fish small amounts, ensuring all food is eaten within a few minutes.
  • Tank Decoration: Use decorations wisely to provide hiding spots without compromising swimming space. This makes your molly fish comfortable while maximizing available room.

Consequences Of Overstocking Molly Fish In Your Tank

Overstocking molly fish in your aquarium can lead to several detrimental effects, including stress, disease, and reduced lifespan.

It’s essential to maintain an appropriate fish-to-tank size ratio for their well-being. Here is what you should know:

  • Stress: Overcrowded conditions can lead to stress among molly fish. Stress not only harms their overall health but also makes them more susceptible to diseases.
  • Disease Spread: When molly fish are overstocked, disease can spread quickly. Stress from overcrowding weakens their immune system, making them more vulnerable.
  • Aggression: Mollies are generally peaceful, but limited space can lead to territorial behaviors. This can result in fights, leading to potential harm or death.
  • Poor Water Quality: More fish produce more waste, affecting the water’s quality. Poor water conditions can lead to various health issues in molly fish.
  • Reduced Lifespan: Stress, disease, aggression, and poor water conditions resulting from overstocking can shorten the lifespan of molly fish.

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

Does The Gender Of Molly Fish Make A Difference?

No, the gender of molly fish doesn’t make a significant difference when it comes to stocking a 10-gallon tank.

There are no substantial size or behavioral differences between male and female mollies that would affect the stocking process.

Can You Breed Molly Fish In A 10-Gallon Setup?

While it is technically possible to breed molly fish in a 10-gallon setup, it’s generally not recommended due to space constraints and the high number of offspring mollies can produce.

Breeding mollies in such a small space could lead to overcrowding and other issues. The reasons include:

  • High Reproduction Rate: Molly fish are livebearers and can produce a large number of fry. A 10-gallon tank may not be able to accommodate this increase in population.
  • Space for Offspring: The fry need space to grow and mature. Overcrowding can lead to stress, disease, and potential cannibalism.
  • Water Quality: With more fish, maintaining water quality becomes more challenging. Poor water conditions can impact the health of both adult molly fish and their offspring.
  • Frequent Breeding Cycle: Molly fish can breed as often as once a month. In a 10-gallon tank, this regular addition of new fish can quickly lead to severe overcrowding.

Adding More Molly Fish To A 10-Gallon Tank: Is It Possible?

If you wish to add more molly fish to a 10-gallon tank, you’ll need to take certain steps to maintain a healthy environment.

However, it’s important to understand that these steps are still a compromise and the situation isn’t ideal for molly fish.

Consider the following:

  • Frequent Water Changes: To maintain water quality, perform more frequent water changes. This helps manage waste buildup and prevent ammonia spikes.
  • Efficient Filtration System: Upgrade to a high-quality filter. This helps keep the water clean and oxygenated, which is crucial when housing more fish.
  • Careful Feeding: Monitor and control feeding carefully. Overfeeding can cause water quality issues, especially in an overcrowded tank.
  • Monitoring Health: Keep a close eye on the health of your fish. Look for signs of stress, disease, or aggression that may result from overcrowding.
  • Consider Upgrading: Plan to upgrade to a larger tank. A bigger tank is the best long-term solution for housing more molly fish comfortably.

Ideal Number Of Molly Fish To Keep Together

The ideal number of molly fish to keep together depends largely on the size of the tank, but generally, a small group of four to five mollies is a good starting point for a tank around 20-30 gallons.

Mollies are social fish that enjoy company, and keeping them in groups helps mimic their natural habitat.

Keeping Molly Fish With Other Species In A 10-Gallon Tank

Naturally, you’d like to keep other fish along with your mollies. Here is a table that includes the most common species that aquarists often consider keeping with them:

Species CombinationNumber of Fish
Only Molly Fish2-3 Mollies
Molly Fish and Guppies1 Molly, 3-4 Guppies
Molly Fish and Neon Tetras1 Molly, 5-6 Neon Tetras
Molly Fish and Zebra Danios1 Molly, 4-5 Zebra Danios
Molly Fish and Platy Fish1 Molly, 2 Platy Fish
Molly Fish and Betta Fish1 Molly, 1 Betta Fish

1. Molly Fish And Guppies

In a 10-gallon tank, you can keep one molly fish and a small group of 3-4 guppies. Both species have similar care requirements and can coexist peacefully.

2. Molly Fish And Neon Tetras

One molly fish and a small group of 5-6 neon tetras could be kept in a 10-gallon tank. 

However, neon tetras prefer cooler water temperatures than mollies, which could pose a challenge.

3. Molly Fish And Zebra Danios

In a 10-gallon setup, one molly fish and about 4-5 zebra danios could coexist. However, zebra danios are very active swimmers and would be more comfortable in a larger tank.

4. Molly Fish And Platy Fish

You can keep one molly fish and two platy fish in a 10-gallon tank. Both species have similar care requirements and generally get along well together.

5. Molly Fish And Betta Fish

Housing one molly fish with a single betta fish in a 10-gallon tank can be done, but care must be taken as bettas can sometimes show aggression towards other fish, especially those with similar appearances.

It’s essential to monitor their interaction closely.


Allow me to briefly summarize what I discussed earlier:

  • The ideal number of molly fish for a 10-gallon tank is around 2 to 3 adult fish to provide enough swimming space and promote their well-being.
  • Calculating the number of molly fish based on the 1-inch-per-gallon rule suggests that a 10-gallon tank can comfortably house 2 full-grown molly fish.
  • The recommended tank size for molly fish is at least 20-30 gallons to allow for ample swimming space and accommodate their social behavior.
  • While a 10-gallon tank may seem spacious, it can quickly become cramped for molly fish due to their size, social needs, and breeding tendencies.
  • When keeping molly fish with other species in a 10-gallon tank, suitable combinations include guppies, neon tetras (with caution), zebra danios, and platy fish.