Do Molly Fish Need A Heater? (And Which One)

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When I was new to fishkeeping, a friend of mine told me that molly fish are hardy and can tolerate extreme conditions.

The next thing I asked him was whether these fish can even live without a heater, although he didn’t really know what to tell me.

Do molly fish actually need a heater? Can they live without one in cold water? In this article, I will answer these questions and share my personal heater recommendation. 

Let’s get started.

Do Molly Fish Need A Heater?

Yes, molly fish require a heater in their tank because they originate from tropical climates, thus needing warm water to thrive.

Mimicking their natural warm habitat within the aquarium ensures they stay healthy, active, and comfortable.

Typically, maintaining a temperature range of 72°F to 78°F (22°C to 26°C) with a heater satisfies molly fish.

Let’s delve into the reasons why molly fish need warm water:

  • Metabolic Dependence: Molly fish rely on warm water for proper metabolism. Too cold, and their metabolism slows, affecting their health and vigor.
  • Optimal Temperature Range: Molly fish thrive between 72°F to 78°F (22°C to 26°C). Deviating from this range can stress molly fish, making them disease-prone.
  • Breeding Success: Warm water enhances molly fish breeding. Colder conditions may hinder their reproduction success.
  • Species’ Longevity: Adequate water temperature contributes to molly fish’s lifespan. Unsuitable conditions can shorten their lives.

Also Read: Molly Fish Tank Setup

What Is The Ideal Water Temperature For A Molly Fish Tank?

Ideally, molly fish should be kept between 72°F and 78°F (22°C to 26°C). As tropical fish, molly fish are adapted to live in relatively warm waters.

Maintaining this temperature range in the aquarium helps to mimic their natural habitat conditions, promoting optimal health, vitality, and breeding success of the molly fish.

Here are some factors supporting the need for this specific temperature range for molly fish:

  • Tropical Origin: Molly fish originate from tropical climates where the water is naturally warm, typically around 72°F to 78°F. This range is best for recreating their native environment.
  • Health and Metabolism: Molly fish’s metabolism functions optimally within the said range. Lower temperatures can slow their metabolic rate, potentially leading to health problems.
  • Breeding Efficiency: The 72°F to 78°F temperature range is conducive for molly fish breeding. Cooler temperatures may decrease breeding success.
  • Disease Resistance: Warm waters help molly fish maintain robust immune systems, potentially lowering their susceptibility to disease.

Also Read: Molly Fish Temperature

Selecting The Appropriate Heater Power For Your Molly Fish

The heater power you’ll need mainly depends on the tank size. Here is a simple table describing just that:

Aquarium Size (Gallons)Heater Power (Watts)

My personal recommendation: Tetra Submersible Heater (link to Amazon):

Choosing the appropriate heater power for your molly fish tank is crucial to maintaining the desired temperature range.

Based on my experience, a useful guideline is to employ a heater with 5 watts of power per gallon of water.

Here are some considerations when selecting the appropriate heater power for your molly fish:

  • Room Temperature: If the room where you keep the aquarium is cool, you may need a heater with higher wattage to maintain the molly fish’s preferred temperature range.
  • Heater Efficiency: Not all heaters are equally efficient. Always choose a reliable brand that guarantees a steady temperature to keep your molly fish comfortable and healthy.
  • Multiple Heaters: For larger tanks, it may be beneficial to use multiple heaters to ensure even heat distribution. This strategy helps avoid creating cold spots, which might stress molly fish.
  • Tank Size: The larger the aquarium, the higher the wattage you’ll need. For instance, a 50-gallon tank typically requires a 250-watt heater to maintain optimal temperature for molly fish.

Also Read: Molly Fish Tank Size

Four Heater Types For A Molly Fish Tank

Generally, you can choose from four types of aquarium heaters for your molly fish tank:

1. Filter Heaters

Filter heaters are a two-in-one solution, providing both heating and filtration for the aquarium. 

They offer efficient heating, evenly spreading warmth throughout the molly fish tank. Their combined functionalities help keep the tank setup neat and space-efficient.

Let’s consider why these features make filter heaters beneficial for a molly fish tank:

  • Efficiency: Filter heaters promote heat distribution evenly throughout the tank, maintaining the ideal temperature range for molly fish.
  • Space-saving: By combining heating and filtration, these heaters minimize the number of devices in the tank, saving space.
  • Reduced Clutter: With fewer cables and devices, filter heaters contribute to a tidier aquarium setup.

2. Substrate Heaters

Substrate heaters are placed under the gravel or substrate in the tank.

These heaters provide bottom-up heating, which can be especially beneficial in planted tanks and also suits the needs of molly fish.

Here’s why substrate heaters can be an excellent choice for a molly fish tank:

  • Even Heating: Substrate heaters provide consistent, evenly distributed heat from the bottom, beneficial for molly fish.
  • Aesthetics: These heaters are hidden under the substrate, providing a cleaner look to your molly fish tank.
  • Ideal for Planted Tanks: For molly fish tanks with live plants, substrate heaters can promote plant growth by heating the roots effectively.

3. Submersible Heaters

Submersible heaters are a popular choice for aquariums, including molly fish tanks. They are fully submerged in the water, providing reliable, adjustable heating.

Below are some advantages of submersible heaters for a molly fish tank:

  • Reliability: Submersible heaters are well-known for their reliability and can effectively maintain the right temperature for molly fish.
  • Adjustable: These heaters often come with adjustable temperature settings, allowing for precise control to keep molly fish comfortable.
  • Versatility: Submersible heaters are versatile and can be positioned in different ways in the tank for optimal heat distribution.

4. Hang-On Heaters

Hang-on heaters are hung on the side of the aquarium. They heat the water as it flows through them, making them a good option for molly fish tanks.

Here are reasons why hang-on heaters can be suitable for a molly fish tank:

  • Hassle-Free Setup: Installing a hang-on heater is a straightforward process, making it an attractive option for heating a molly fish tank.
  • External Mounting: These heaters are affixed on the exterior of the tank, hence, they don’t consume space inside the molly fish tank.
  • Optimal Heat Dispersion: As the water circulates through them, they heat it, ensuring a uniform distribution of heat across the entire molly fish tank.

What If Molly Fish Are Kept In Extremely Cold Water?

If molly fish are kept in extremely cold water, it can significantly impact their health and overall well-being.

Molly fish are tropical creatures and cold water can slow their metabolism, resulting in reduced activity, lower eating rates, and potentially weakened immune systems.

In severe cases, consistent exposure to extremely cold temperatures can lead to illness and even death for molly fish.

Here’s a deeper look at what can happen if molly fish are subjected to extremely cold water:

  • Reduced Metabolism: Cold water temperatures can drastically slow down the metabolism of molly fish. This can affect their ability to process food and extract necessary nutrients, leading to overall poorer health.
  • Lower Activity Levels: Molly fish kept in cold water may become lethargic. Their movement may slow down, and they may show less interest in feeding, both of which are signs of stress in fish.
  • Weakened Immune System: The stress caused by cold water can weaken molly fish’s immune systems, making them more susceptible to disease.
  • Risk of Death: In severe cases, prolonged exposure to cold water can lead to fatal conditions for molly fish, especially if their immune system is significantly weakened and they develop diseases.

What If Molly Fish Are Kept In Extremely Hot Water?

If molly fish are kept in extremely hot water, it can lead to serious health issues and potentially be fatal.

This is because mollies have a certain optimal temperature range and any deviation from this range, especially towards higher temperatures, can severely impact their metabolic and physiological functions.

Consider the following:

  • Oxygen Depletion: Warm water holds less dissolved oxygen. In extremely hot conditions, mollies may suffocate due to a lack of oxygen.
  • Accelerated Metabolism: High temperatures increase molly fish’s metabolism, causing overconsumption and overproduction of waste, which strains their systems.
  • Immune System Suppression: Extreme heat can suppress the mollies’ immune system, making them more susceptible to diseases and infections.
  • Behavioral Changes: Mollies may exhibit lethargic behavior or unusual swimming patterns under heat stress, indicating health issues.

What If Molly Fish Experience Rapid Temperature Changes?

Rapid temperature changes can be quite stressful for molly fish, potentially leading to health problems.

Temperature fluctuations can weaken their immune system, making them more susceptible to diseases.

It may also lead to unusual behavior, lack of appetite, or in severe cases, death. 

Can I Keep Molly Fish Outdoors?

Yes, you can keep molly fish outdoors, but careful attention must be given to maintaining the appropriate temperature range.

In areas where the temperature consistently falls within the recommended range, molly fish can be kept outdoors.

However, in regions with colder seasons or unpredictable temperature swings, additional steps may need to be taken to keep your molly fish healthy and comfortable.

Here are some temperature-related points to consider:

  • Seasonal Changes: Some regions have wide temperature swings across seasons, which could harm molly fish. If you live in such an area, it may be necessary to move your molly fish indoors during colder seasons.
  • Temperature Regulation: Tools such as heaters or chillers can help maintain the ideal temperature range for molly fish in an outdoor setup. It’s important to monitor the temperature regularly to make necessary adjustments.
  • Night and Day Fluctuations: Temperature can vary significantly between day and night. Consider how you might mitigate this to avoid stressing your molly fish.
  • Sunlight Exposure: Direct sunlight can rapidly increase water temperature. Ensure your outdoor pond or tank has areas of shade to prevent overheating.
  • Insulation for Cold Weather: If the temperature dips below the comfortable range for molly fish, you may need to insulate your outdoor pond or use a pond heater.


For those of you who hurry, here is a brief summary of what I discussed above:

  • Molly fish require a heater in their tank to recreate their natural warm habitat, ensuring their health and activity levels.
  • Maintaining a temperature range of 72°F to 78°F (22°C to 26°C) with a heater satisfies the needs of molly fish, promoting their metabolic function and breeding success.
  • Selecting the appropriate heater power for a molly fish tank depends on factors such as tank size, room temperature, and heater efficiency.
  • Four types of aquarium heaters – filter heaters, substrate heaters, submersible heaters, and hang-on heaters – offer different advantages for maintaining the desired temperature in a molly fish tank.
  • Extreme cold or hot water can significantly impact the health and well-being of molly fish, affecting their metabolism, activity levels, and immune system, potentially leading to illness or death.