Do Mollies Need A Heater? Is It Necessary For Their Survival?

If you have built a fish tank on your own, then you must be no stranger to mollies. Being part of the Poeciliidae family, mollies are classified as tropical freshwater fish. Best known for their adaptability and various color tones, mollies are the best thing you can give to a newbie aquarist.

That said, it does not mean that taking care of mollies is a piece of cake. In today’s article, we will explore the bugging question of “Do mollies need a heater?”, and learn more about this species!

Do Mollies Need A Heater?

In most cases, mollies do need a heater to survive and thrive.

This is because mollies come from a hot climate, where the temperature remains relatively high. When thrown out of their original habitat, mollies may have a difficult time acclimating themselves.

In case you wish to ensure a high survival rate for mollies, replicating how their living environment used to be is a logical course of action.

But still, it is worth remembering that a heater is only needed should you live somewhere cold. There is no way your indoor or outdoor tank can manage to provide warm water for mollies when the outside temperature drops below 20 degrees Celcius.

On the other hand, aquarists living in tropical climates should not worry much about this problem. As long as the room temperature stays warm and suitable enough for mollies, investing in a heater will not be necessary.

Now, some people might be wondering why mollies even need somewhere warm. Isn’t the whole point of having mollies is that they are easy to feed and take care of?

Admittedly, mollies are versatile and can pretty much live anywhere within their tolerant capacity. However, mollies can only reach their full potential if the surrounding environment is designed to their liking.

What’s The Ideal Temperature For Mollies?

Most experts agree that the best temperature for mollies should be the same as any typical water body from tropical lands. This means the acceptable range falls between 21 to 30 degrees Celcius, which is equivalent to 70 degrees Fahrenheit and 86 degrees Fahrenheit respectively.

And remember, this is not the room temperature that we are talking about! This is the temperature of your fish tank, so make sure to use a thermometer to check things up.

How To Keep Molly Fish Indoors

Mollies are not picky when it comes to light exposure. Thus, it does not matter if you keep them indoors or outdoors. Still, there are some guidelines to follow in case your mollies are kept under your roof.

The most important thing you have to remember is the temperature. As stated above, you do not need a heater should your house resemble a tropical climate. But if your place is suffering from a cold spell, you will need a separate heater beside the overall heater.

Why? Because a home heater cannot do much when it comes to a deep body of water like your fish tank. Liquid requires more time to get heated up, which means the aquarium may end up being colder than the room temperature.

If left in cold water for a long time, your mollies will be more susceptible to diseases.

How To Keep Molly Fish Outdoor

Some people prefer the feeling of having an outdoor pond and do not want to confine their mollies to a small tank inside the house. For those living in a tropical climate, this should be easy enough.

Since the weather stays warm and comfortable throughout the year, the only thing you should be worried about is when the heat strikes. Should the weather become blazing hot, it would do your mollies good to be transferred temporarily. Otherwise, you will not have much trouble taking care of the fish.

But for anyone living where the temperature can drop surprisingly low, especially in the winter, having mollies kept outdoors is not as favorable.

Again, they will have to be moved inside to prevent any untimely deaths.


1. What happens if the water becomes too cold for mollies?

Since mollies originate from a tropical climate, having to live somewhere cold will negatively affect their development and well-being.

Studies have shown that mollies living in cold water cannot grow at all. As time goes by, they will slow down and become extremely lethargic. At some point, mollies will turn into lifeless creatures and die.

Furthermore, the cold temperature will get to mollies and make them more stressful. The pressure of having to swim in some strangely uncomfortable water bodies tends to put a burden on their mind, even when they are not fully aware of the impact.

But rest assured that mollies will notice the difference and act accordingly, which usually translates to a more sluggish lifestyle and a shorter lifespan.

Last but not least, the inability to regulate their body temperature is something directly linked with mollies’ vulnerability to diseases. When the water is too cold, their entire organs turn cold as well. Thus, it creates more opportunities for parasites or other diseases to take root.

2. What happens to mollies if the water temperature drops suddenly?

A sudden drop in temperature can be detrimental to mollies. Imagine the effect of cold water on mollies, and then multiply it by 10 times! Mollies can easily fall prey to diseases the moment they experience such a shock.

Furthermore, their reproductive system is also compromised, leading to fewer baby mollies being born. Even if mollies manage to spurt out some offspring, chances are these fish will die due to prolonged exposure to coldness.


Do mollies need a heater? In most cases, having a heater ensures their survival rate and helps them remain healthy. Hence, you are highly recommended to get a heater should you want your mollies to stay for a long time!