Do Mollies Need a Heater to Survive?

As tropical freshwater fish, mollies enjoy warmer water. Their optimal temperature range is between 70-82 °F (21-28 °C).

Besides requiring warmer water, mollies also need stable temperatures as any sudden changes in temperature levels will have an adverse impact on their health.

Whenever I am asked “Do I need a need a heater for my molly fish?”, I try to explain that it’s unlikely that you can maintain temperature at optimal levels in your tank without a heater.


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Therefore, unless you live in a tropical climate, the answer is always: “Yes, you do need a heater for your molly fish tank.” I recommend the Eheim Jager aquarium heater, which you can buy from Amazon.

An aquarium heater is the easiest way to maintain temperature stable in the tank and prevent sudden drops in temperature, which can make your mollies sick.

In this article, I discuss the rule of thumb for choosing a heater as well as managing temperature in indoor vs outdoor setups, and the effects of inadequate temperature conditions.

Heater Size for Molly Aquarium – How to Calculate?

Aquarium heaters are rated in watts and the wattage of your heater should be determined by the size of your aquarium.

The time-tested rule when it comes to sizing the aquarium is to calculate 5 watts per gallon of water volume. Therefore, if you’re setting up a molly fish tank of 25 gallons, you’ll need a 125 watts heater.

Tank Size Heater Wattage
5 Gallon / 20 Liter 25 W
10 Gallon / 40 Liter 50 W
20 Gallon / 80 Liter 100 W
25 Gallon / 95 Liter 125 W
40 Gallon / 150 Liter 200 W
50 Gallon / 190 Liter 250 W
65 Gallon / 250 Liter 2 X 200 W
75 Gallon / 285 Liter 2 x 300 W

By observing this rule, you’ll be able to heat the water in your aquarium to match the temperature range required by your mollies.

Invest in an aquarium heater that comes with a thermostat, this way the temperature of your aquarium won’t be affected by changes in your room temperature.

I highly recommend the Eheim Jager aquarium heater from Amazon. I have couple of this heaters in my fish room for more than 5 years and still work perfectly.

Keeping Molly Fish Indoors

As I mentioned, you can get away with keeping molly fish in indoor aquariums without a heater if you live in a region with tropical climate, where temperatures don’t drop below 68-70 °F.

How about if you don’t live in a tropical climate but you’re heating your home when temperatures drop? Do you still need a heater in a well heated home?

My answer is still the same – Yes. The temperature in your room may be fine, but the water in your aquarium won’t have the same temperature as the air in your home.

The reason is simple – water heats up and cools slower than air, so a heater is indispensable for reaching the required temperature levels.

The problem with not getting a heater is that you can’t heat up the water otherwise or keep it at healthy levels for your mollies.

This will lead to a series of health issues for your mollies that will ultimately end in the untimely death of your beloved fish.

If the health of your molly fish depends on something as simple as getting a heater, there is no good reason why you shouldn’t get one for your aquarium.

Therefore, even if you keep your mollies in a well-heated home, you still need to invest in an aquarium heater for your fish tank.

Keeping Molly Fish in Outdoor Ponds

What if you are not keen on keeping your mollies indoor, and you’re planning on setting up an outdoor pond for your fish. What should you do?

If you live in a tropical climate, where temperatures in the winter stay above 68 °F, you can go ahead and keep mollies in an outside tub or pond.

What if you don’t live in a climate with warm weather all year round? What if you live in an area with temperatures below freezing level in the winter? Can you keep molly fish outside?

Obviously, no. At least not during the winter. However, there is no reason why you shouldn’t keep molly fish in an outdoor pond during the summer.

Daytime sun will heat up the water in the pond, which won’t drop significantly during the night. If there is no shade around the pond, you may want to create some shade to avoid water overheating when sunlight is intense.

When temperatures start to drop in the fall, your molly fish need to be moved inside. You may experiment with heating systems, but the truth is that it’s best to keep your mollies inside during the fall, winter and beginning of spring.

As soon as outside temperature no longer drops below 68 °F, you can set up the outside ponds for your molly fish.

Health Effects of Inadequate Water Temperature

Temperature is very important to fish, because they’re inefficient thermoregulators. In the wild, fish behaviorally regulate their body temperature, which means they seek out waters with temperatures that optimize their physiological performance.

In an aquarium, fish cannot seek out regions with thermal regimes suitable for them, therefore, they’re completely reliant on the thermal regime you provide them.

Therefore, it’s crucial that you research the optimal temperature levels required by your fish and strive to recreate it by using a heater if temperature conditions in your area aren’t otherwise suitable for your fish.

Sudden drops in temperature can compromise the immune function of your molly fish and give way to opportunistic parasites and bacteria to thrive.

Susceptibility to diseases in fish is correlated with thermal stress. Fish that are exposed to unfavorable temperature conditions experience reproductive problems, susceptibility to infections, and higher mortality rates.


Molly fish are viewed as hardy fish by the aquarium community and even though they can tolerate a wide range of temperatures, they require stable conditions to thrive.

An aquarium heater is the simplest and most efficient way to ensure that the temperature in your molly fish tank stays at the optimal level.

Keeping the temperature at the value required for your molly fish is challenging in regions where winter comes with below freezing temperatures.

Therefore, if you don’t live in a region with tropical climate, do yourself a favor and invest in a quality heater for your aquarium. Your fish will be healthy, and you’ll save yourself a lot of trouble down the road.

I hope my article has offered you valuable information on how to size your aquarium heater and how to manage temperature conditions in indoor aquariums and outside ponds.

Questions and Answers

    You can keep mollies all the time in 28 °C water. They will probably grow much faster, but will live less.

Thank you for all the information. I have mollies outside in a tub.ive separated the babies to another tub.all are thriving happily.will take them indoors when it’s cold.

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