Molly Fish Temperature: A Complete Guide

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Keeping molly fish is pretty straightforward, as they are hardy fish that can tolerate a wide range of situations and conditions.

However, keeping their temperature at the desired range may raise some questions. What temperatures do they like? And what happens if the water gets too hot or too cold?

In this article, I will answer all these questions and many more. Let’s dive right into it.

Ideal Water Temperature For Molly Fish

Molly fish thrive best in water temperatures ranging from 72 to 78 degrees Fahrenheit (22 to 26 degrees Celsius).

Maintaining this temperature range is crucial to their health and overall well-being.

Let’s discuss the importance and implications of this temperature range for molly fish:

  • Optimal Living Conditions: Molly fish originate from warm, tropical regions where water temperatures naturally fluctuate between 72 and 78 degrees Fahrenheit. These conditions mimic their natural habitat, leading to happier, healthier fish.
  • Health and Disease Prevention: When the water temperature for molly fish is kept within this range, it can help prevent diseases. Sudden temperature changes or temperatures outside this range can stress molly fish, making them more susceptible to illness.
  • Promotion of Healthy Growth and Reproduction: Molly fish kept within the optimal temperature range are more likely to grow to their full potential and reproduce successfully. Temperature can influence metabolic rates and, subsequently, growth and reproduction rates.
  • Impact on Metabolism: Just like other fish, the metabolism of molly fish is regulated by water temperature. Temperatures in the aforementioned range ensure their metabolism functions efficiently, supporting energy levels and overall health.

Also Read: Molly Fish Tank Setup

What Is The Minimum Water Temperature For Molly Fish?

The minimum water temperature for molly fish should not drop below 70°F (or about 21°C). 

Below this threshold, the health of molly fish can be severely compromised, affecting their activity levels, growth, and overall wellbeing.

Here are the reasons detailing the importance of this minimum temperature:

  • Temperature Tolerance: Molly fish are tropical fish species, originating from warmer climates. A minimum water temperature of 70°F is essential to match their natural living conditions.
  • Metabolic Impact: The metabolism of molly fish slows down significantly below this temperature. Slower metabolism affects their digestion, growth rate, and overall vitality.
  • Disease Vulnerability: Colder water can stress molly fish, making them more susceptible to illnesses. Maintaining the minimum temperature helps ensure their immune systems operate effectively.
  • Behavioral Changes: Molly fish become less active and display changes in their normal behavior when the water is too cold. This is a clear sign that they are not comfortable or healthy in their environment.

Effects Of Cold Water On Molly Fish

While molly fish can survive in cold water, it surely isn’t optimal for them. Consider the following:

1. Impaired Growth

Cold water can impede the growth of molly fish.

Molly fish, being tropical species, require warmer temperatures for their metabolic functions to work optimally, and this includes growth:

  • Metabolic Slowdown: Colder water slows down the metabolism of molly fish. A slow metabolism means the food isn’t processed as efficiently, resulting in less energy for growth.
  • Energy Diversion: When in cold water, molly fish’s energy is used for survival rather than growth. Energy that would typically be allocated to growth gets diverted to maintaining basic bodily functions.
  • Food Intake Reduction: Molly fish tend to eat less in colder temperatures. Reduced food intake means less nutrients available for growth.

2. Inhibited Breeding

Reproduction in molly fish can be negatively affected in colder water.

Optimal breeding conditions for these fish are found in warmer waters, and a decline in temperature can result in a reduction in breeding activity:

  • Mating Behavior Reduction: Molly fish are less likely to exhibit mating behaviors in cold water. This limits the chances of successful breeding.
  • Egg Production: The production of eggs in female molly fish can decrease in colder temperatures, contributing to less successful breeding.
  • Embryo Development: Even if breeding occurs, cold water can slow down or impair the development of embryos, leading to a reduced survival rate.

3. Increased Vulnerability To Diseases

Cold water can stress molly fish, weakening their immune systems and making them more susceptible to diseases:

  • Stress Impact: Chronic cold stress can lead to a weakened immune response in molly fish, making them more vulnerable to infections and diseases.
  • Parasite Proliferation: Parasites that affect molly fish can proliferate more easily when the fish’s immune system is compromised due to cold stress.
  • Infection Symptoms: The effects of infections or diseases in molly fish may become more noticeable and severe in colder water due to their lowered immune response.

4. Decreased Activity

The activity level of molly fish drops in cold water. This is an adaptive response to conserve energy but also a sign of discomfort and stress:

  • Survival Mechanism: Decreased activity is a survival mechanism in molly fish, helping them to conserve energy in an unsuitable cold environment.
  • Feeding Behavior Changes: Molly fish may feed less frequently and show less interest in food in cold water due to their decreased activity.
  • Social Interaction Reduction: Social behaviors, such as schooling or interacting with other fish, may also decrease as molly fish become less active in cold water.

What Is The Maximum Water Temperature For Molly Fish?

The maximum temperature molly fish can tolerate is approximately 82 degrees Fahrenheit (28 degrees Celsius).

While molly fish can survive in these temperatures, it is not the ideal condition for them. Let’s delve deeper into the significance of these temperature parameters for molly fish:

  • High Temperature Tolerance: Molly fish can survive up to 82 degrees Fahrenheit, although this is the upper limit. Prolonged exposure to such high temperatures may induce stress in these fish and potentially harm their health.
  • Behavioral Changes: When water temperatures are consistently near or at the high end of the scale, molly fish may exhibit signs of discomfort. These can include lethargy, reduced appetite, and less interactive behavior, signaling that the environment is not ideal.
  • Health Implications: Extended periods in maximum temperature conditions can weaken the immune system of molly fish. This stress-induced vulnerability can make them more susceptible to diseases and can potentially shorten their lifespan.

What To Do If the Water In Your Molly Fish Tank Is Too Hot

If the water in your molly fish tank is too hot, there are several measures you can take to fix this:

1. Enhance Gas Exchange

Increasing the rate of gas exchange in the water can be an effective method to help cool the water.

This process, which occurs when water and atmospheric air exchange dissolved gases, is promoted by movement on the water’s surface.

Now let’s delve deeper into the specific ways to improve gas exchange:

  • Use of Air Stones: Air stones can be placed in the tank to generate bubbles. These bubbles create surface agitation, promoting increased gas exchange and aiding in cooling the water.
  • Employ Powerheads: These devices are beneficial in creating water movement. By enhancing circulation, powerheads facilitate a higher rate of gas exchange, helping to reduce the water’s temperature.
  • Leverage Water Filters: Some types of water filters, such as Hang-On-Back (HOB) filters, can be useful. These filters return water to the tank in a way that causes surface agitation, which improves gas exchange and consequently cools the water.

2. Improve Cooling With A Fan

Using a fan to increase evaporation from the water’s surface is another efficient method to cool the tank.

The fan helps to enhance the evaporation process, resulting in a natural cooling effect on the remaining water in the tank.

Let’s explore this in more detail:

  • Fan Position: Position the fan to blow across the water’s surface. By maximizing the water surface area exposed to the air, this process effectively increases evaporation and enhances the water cooling process.
  • Fan Size: Choose a fan size that’s suitable for your tank size. Larger tanks may require bigger or multiple fans to achieve sufficient cooling.
  • Regular Monitoring: Be sure to monitor the temperature regularly when using a fan to ensure you don’t overcool the tank and stress your molly fish.

3. Use An Ice Bag

In critical situations with exceptionally high temperatures, employing an ice bag can swiftly lower the temperature as a viable solution.

Let’s examine the key points:

  • Proper Use: Always float the ice bag in the tank water. Directly adding ice can cause a shockingly rapid temperature change, which is harmful to your molly fish.
  • Temporary Solution: While effective in the short term, using an ice bag is not a permanent solution. Always work to establish a stable, suitable temperature through other means.
  • Monitor Temperature: It is essential to carefully monitor the water temperature when employing this method to prevent excessive cooling of the tank.

4. Perform Water Changes

Regularly changing a portion of the water can help manage the temperature in your molly fish tank. But there are some critical points to remember:

  • Cool Water: Use cool (but not cold) water when replacing a portion of the tank water. This can gradually reduce the temperature without shocking your molly fish.
  • Routine Changes: Implement a routine for water changes to maintain a consistently appropriate temperature.
  • Avoid Rapid Changes: Rapid temperature swings can stress molly fish. Always ensure changes are gradual.

5. Adjust Heater Settings

Consider adjusting the settings of the heater if you have one installed. This can prove to be an effective approach:

  • Correct Settings: Ensure your heater is set to the appropriate range for molly fish, which is 72-78 degrees Fahrenheit (22-26 degrees Celsius).
  • Regular Checks: Check the function of your heater regularly to avoid accidental overheating.
  • Quality Equipment: Use a reliable, high-quality heater to help maintain a consistent water temperature.

Also Read: Do Molly Fish Need A Heater?

6. Turn Off The Lights

If your tank lights are contributing to the heat, consider turning them off for periods. However, this tactic should be employed wisely:

  • Reducing Heat: Certain types of lights, especially incandescent lights, produce significant heat. Turning them off when not necessary can help reduce overall tank temperature.
  • LED Lights: Consider using LED lights for your tank, as they produce less heat compared to other lighting types.
  • Light Cycle: Always maintain a proper day-night light cycle for your molly fish, even when attempting to reduce heat.

Do Molly Fish Need A Heater?

Yes, a heater is generally considered necessary for molly fish, especially if you’re unable to maintain a consistent water temperature within their ideal range.

Fluctuations in temperature can stress molly fish and lead to health issues.

A heater helps maintain a steady, comfortable water temperature for molly fish, promoting their health and well-being.

Here’s how a heater can be beneficial for a molly fish tank:

  • Maintaining Consistent Temperature: An aquarium heater keeps the water temperature consistent, which is essential for the health of molly fish. Abrupt changes or inconsistent temperatures can lead to stress and disease.
  • Mimicking Natural Environment: Molly fish are tropical fish and thrive in warm water. A heater helps recreate their natural environment, allowing them to thrive.
  • Preventing Disease: Consistent and suitable water temperature can help prevent diseases, which often result from stress due to fluctuations in temperature.
  • Enhancing Metabolic Processes: Molly fish metabolism functions optimally at certain temperatures. A heater maintains these temperatures, ensuring healthy growth and activity.

My recommendation: Orlushy Submersible Aquarium Heater (link to Amazon):

How To Maintain Water Temperature During Water Changes For Molly Fish

To maintain water temperature during water changes for molly fish, it’s crucial to ensure the new water is the same temperature as the existing tank water.

This is important because sudden temperature changes can stress your molly fish. Here’s how to ensure temperature consistency during water changes:

  • Use a Thermometer: A reliable thermometer can accurately measure the temperature of both the tank water and the new water, ensuring they match and prevent sudden temperature changes.
  • Regulate New Water’s Temperature: If the new water is colder, warm it up to match the tank’s temperature; if it’s warmer, allow it to cool. If needed, you can modify the temperature by allowing the water to settle at ambient temperature or utilizing an independent heater.
  • Gradual Water Addition: Add the new water slowly into the tank to avoid abrupt temperature changes. Also, closely monitor the tank’s temperature during this process to ensure consistency.

Can Molly Fish Be Kept In An Outdoor Pond?

Yes, molly fish can be kept in an outdoor pond if the water temperature can consistently stay within their preferred range of 72-78 degrees Fahrenheit (22-26 degrees Celsius).

However, in climates with significant temperature fluctuations or cooler temperatures, keeping molly fish outdoors may not be ideal.

Optimal Water Temperature For Breeding Molly Fish

The optimal water temperature for breeding molly fish is slightly warmer than their usual range, typically around 78-82 degrees Fahrenheit (25.5-27.8 degrees Celsius).

Warmer temperatures encourage breeding behavior and enhance the overall health of the fry, ensuring better survival rates. 

When Is It Suitable To Expose Molly Fish To Warmer Water?

There are situations where exposing molly fish to slightly warmer water can be beneficial:

  • Breeding Encouragement: As mentioned earlier, increasing the water temperature to between 78-82°F (25-28°C) can stimulate breeding behaviors in molly fish.
  • Illness Recovery: If a molly fish has been ill, slightly raising the tank’s temperature (within their comfortable range) can help speed up their metabolism, boosting the fish’s immune system and aiding in faster recovery.
  • Acclimation Support: When introducing new molly fish to an aquarium, it’s crucial to slowly increase the water temperature to match that of their previous habitat.

How To Position A Molly Fish Aquarium To Prevent Temperature Fluctuations

To maintain a stable temperature in a molly fish aquarium, place it in a spot that avoids direct sunlight, drafts, and heat sources.

This will help ensure a consistent and comfortable temperature for your molly fish. Here are the important considerations:

  • Avoid Direct Sunlight: Sunlight can cause rapid temperature changes, so avoid areas with direct sunlight to protect your molly fish.
  • Steer Clear of Drafts: Windows, doors, and AC vents can cause drafts, leading to quick temperature drops. Keep the molly fish tank away from these areas.
  • Distance from Heat Sources: Heaters or radiators can overheat the tank. To avoid this, don’t place the molly fish aquarium near these sources.
  • Consider Room Temperature: Choose a room with a stable temperature to further ensure consistency and comfort for your molly fish.

Additional Water Requirements For Molly Fish

Besides the ideal temperature for molly fish, you should also take into considerations the pH level and water hardness:

1. pH Level

The best pH level for molly fish is slightly alkaline, ideally between 7.5 and 8.5.

This range closely replicates the pH of their natural habitats and supports their overall health and well-being.

Also Read: What Is The Best pH Level For Molly Fish?

2. Water Hardness

Molly fish thrive in hard water environments, with the ideal hardness ranging between 10 and 25 dGH (degrees of General Hardness).

They are adaptable to a certain extent but do best in these conditions as it mirrors their natural habitats.


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

  • Maintaining water temperature of 72-78°F (22-26°C) is crucial for molly fish’s health and well-being.
  • Cold water below 70°F (21°C) can impair growth, breeding, increase disease vulnerability, and decrease activity.
  • High temperatures above 82°F (28°C) can induce stress, behavioral changes, weaken the immune system, and harm molly fish.
  • Measures to maintain ideal temperature: enhance gas exchange, use fans for evaporative cooling, employ ice bags (temporarily), perform gradual water changes, adjust heater settings, and avoid heat sources.
  • A heater is necessary for molly fish tanks to prevent stress, disease, and fluctuations in temperature.