Do Mollies Need a Heater? What’s The Ideal Temperature

Are you thinking of buying a heater for your molly fish? I would suggest that you go for it. The reason why mollies need heaters in their tanks will be further discussed in this article, along with what the optimum temperatures are to keep your molly fish alive and well. By the end of this article, you will understand why you might need a heater for your molly fish.

Molly fish are freshwater tropical fish that do well in hot weather. Therefore, to give your pet a safe environment, you must have heaters if you live somewhere where it gets colder frequently.

Molly fish, as we know, are cold-blooded species. This means that they cannot regulate their body temperature on their own. This is why they need to be in an environment that best suits their needs to stay healthy and well.

Do Mollies Need a Heater

These little beauties can be found in freshwater, in tropical climates. Being adjusted to the tropical environment, they can withstand a wide range of temperatures; therefore this temperature needs to be stable. Molly fish can thrive well in the temperature range of 70-85 °F, or approximately 21-29 °C.

Being able to survive such a wide temperature makes them easier to keep as pets. I myself find them rather easier to maintain and tend to. However, as easy as it may seem, they are also delicate creatures in an environment that they are not suited to, such as harsh temperature conditions. So, gentle care must be taken if you are not keeping them in their natural habitat. Hence, it is safe to say that molly fish do need a heater. I will further highlight why that is.

What Happens When Molly Fish is not kept at a Suitable Temperature?

What if the Water is Too Cold?

1. Stunted Growth

Those who already have mollies will know that these fish are generally small in size. They can grow up to 5 to 6 inches over their lifetime. If they are taken in as pets as babies, and the water temperature is far too low for them, they will not grow to their average size and may stop growing after they have reached about 3 inches.

2. Stress

These tiny organisms cannot handle the harsh cold that well and end up being stressed by trying to escape the cold water. Their bodies are not made to regulate their body temperature according to their needs, and they end up being cold and restless all the time. Stress can be extremely dangerous and deadly for them.

3. Less Mobility

As happens with many other living beings, cold water that is below their range of temperature can cause them to become less mobile.

4. Diseases and Micro-organisms

Water at lower temperatures can give birth to certain microorganisms and parasites. These parasites feed on the molly fish and can cause diseases in them, eventually killing them.

5. Inability to Reproduce

Cold water temperatures and sudden drops in temperature can make the molly fish unable to reproduce. Not only does cold kill them, but also they do not grow in number, eventually causing you to lose all your mollies.

What if the water is Too Hot?

1. Stress

As the water temperature around the molly fish increases, far above their optimal range of temperatures, they begin to feel stressed. They are not able to keep themselves cold enough, and this stress becomes deadly for them.

2. Oxygen level Decreased

As the water temperature exceeds 32 °C, the plants in the aquarium produce less oxygen. Also, the oxygen content of the water itself decreases. This results in lesser oxygen for the molly fish to breathe in, thus leading to their death.

3. Bacteria

Warm water causes ammonia to build up, which in turn causes bacteria to form which can result in diseases in the molly fish.

What if the Temperature Changes Suddenly?

As I have discussed earlier that molly fish are cold-blooded animals, they do not have the mechanism to adjust their body according to their surrounding water temperature. Sudden changes in water temperatures, such as temperature falling below the optimal range, or rising above it, too soon, can be harmful to your molly fish. Their bodies are misled about their surrounding temperature if it changes suddenly, and their inability to adjust to it can cause them to be restless and disturb them to the point that they might not be able to survive. The sudden changes can affect their immune systems, making them more prone to bacteria and diseases.

What’s The Ideal Temperature For Mollies

How to Maintain the Water Temperature for Molly Fish?

I am sure that by now, you must be convinced as to why your molly fish need a heater to survive. People still ask if they should buy a heater if they live in a tropical climate, which is the natural habitat of the molly fish. Well, that can be an exception to needing heaters, as the water temperature is exactly within the range that these fish need. However, not all fish keepers are living in the natural habitat of a molly fish, so what to do? Here are some specifications of how to cater to molly fish, using a heater under different weathers and climates.

Tropical Climates

In these regions, molly fish can be easily kept in an outdoor pond, which meets their temperature needs. Do note that even in these climates, it is recommended to keep the mollies in an outdoor environment. This is because we have indoor heating and cooling systems in our homes and keeping molly fish in an aquarium inside the house means bringing them away from that tropical climate. So, if you live in tropical areas, and wish to keep your molly fish in an aquarium inside your house, it is best to arrange a heater for them.

Tropical Climates

Good plantation around the outdoor pond for molly fish is highly recommended as the sun in the morning may cause the water temperatures to rise above optimum.

Cold Climates

In areas where the temperatures are below 20 °C, it is highly recommended that a heater is kept in the molly fish tanks. As the temperature of the water may change as per the surroundings, even with a heater, it is suggested that a heater is bought with a thermostat. This detects the changes in temperature and helps the heater regulate the temperature back to the set norms.

Cold Climates

If the areas that you live in are around the temperatures of 21-29 °C during summers, you can keep your molly fish outside for that season. This would still mean that you need a heater during the other seasons. You can even put a heater in your outdoor pond, to ensure that the molly fish is well within their temperature range.

Hot Climates

Even if molly fish thrive well in tropical climates, a temperature too hot for them can be deadly. For temperatures exceeding 30 °C, molly fish can be at risk. It is wise to keep your molly fish in an aquarium or tank inside your house if you are living under hot weather conditions. However, as discussed earlier, the room temperature that you set for yourself using air conditioning, may not be suitable for your molly fish, and may even differ from the water temperature of your tank. It may drop below the optimum temperature for your fish. So, it is wise to buy a heater for your molly fish.

Hot Climates

Heater Specifications

Sometimes many people ask me how I bought my aquarium heater. Mainly, they are confused about the heating capacity of the heater versus the water tank. Before buying my heater, I did my research and discovered a suitable ratio of heater power to tank size.

It is recommended that if your tank is 5 Gallons in volume, you buy a heater with a 25W power rating. This means that for every 1 Gallon volume of the tank, a heater with a power rating of 5 W is sufficient to keep it warm. I have summed this up in the table below:

The volume of Tank/ GallonsPower Rating of Heater/ Watts

The size of your heater entirely depends on the size of your tank. When it comes to choosing the right heater for your molly fish, make sure that the heater that you are buying, works in the ranges of temperatures suited for your molly fish. Also, a thermostat and a heater with a thermometer are recommended as it helps you keep checking if the heater isn’t working or if the temperature falls or rises out of the optimal range.

A typical 100W aquarium heater will cost you within the range of $10 to $60, which is convenient if you wish to keep your fish healthy.

The Bottom Line On Do Mollies Need a Heater?

By now, you must be convinced to buy a heater for your molly fish. Even if you live in a tropical environment, having a heater in your fish tank or pond is a safety measure to keep the molly fish well within their required temperature range.

I have arranged a heater for my mollies even though I live in a relatively hot climate. This is because even if the outside temperature is hot enough, the temperature of the water in my molly fish tank is relatively cold due to the air conditioning system of my room. Therefore, a heater is required to keep the water warm. Moreover, the sudden changes in the room temperature make it absolutely necessary to keep a water heater for my mollies.

If you have come this far in the article, I would suggest that you quit thinking any further and go buy that heater that your molly fish need, and keep your molly fish healthy, warm, and cozy!

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