Do Mollies Need A Heater? (And What’s The Ideal Temperature)

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Last Updated on 2023-07-31

The Molly fish is a freshwater fish that belongs to the Poeciliidae family. It originates from Northern and Southern America, particularly in tropical climates.  

Although classed as freshwater fish, Mollies can be found in brackish water conditions as well as marine waters. 

Because the Molly fish originates from a tropical climate, there will be some things you need to know to keep them well maintained. For example, in ‘Do Mollies need a heater’ and ‘What temperatures are ideal for Mollies,’ this article explains everything you need to know to keep these fish acclimated and the risks of extreme temperatures. 

What’s The Ideal Temperature For Mollies?

These tropical fish aren’t used to colder climates, so it is important to ensure they are warm enough all year round! 

Furthermore, observing Molly’s in the wild, we know that they can live in varied conditions. They can adapt to different water types. However versatile this fish may be, it is important to remember that although the water consistency may change, the temperature they are found in does not alter too much. 

With that said, there are some suggestions that Mollies could be acclimated to if you are thinking of going through that process. If you do decide to acclimate your Mollies, it is at your own discretion, and this article provides information on what will happen to the fish in colder temperatures. 

Ideally, mollies survive best in temperatures that are similar to where they are originated from. This means anywhere between 71-82F. Remember, these are water temperatures and not room temperatures! 

Now, if you live in these climates, then that is great news! However, you need to think of where the tank is placed in your house. Is the room usually quite hot or cold? This can affect the tank’s temperature also, so although you may have these temperatures outside, they may not correlate within your home.  

You may be more inclined to have Mollies as outdoor fish in these regions. 

Alternatively, if you live in colder climates, you will need to pay more to ensure your tank is at the right temperature year-round.  

swordtail and molly | Kai Schreiber | Flickr

Do Mollies Need A Heater?

As Mollies are freshwater fish that are found in tropical climates, they will need a heater. The only exception to this is if you already live in a place that maintains hot climates. Mollies are a pretty versatile fish; however, as we previously stated, the best temperature would be between 21 degrees Celsius and 29 degrees celsius.  

This means that if you are living in colder climates that are not achieving these temperatures year-round, you will need to invest in some good heaters for your Mollies. These heaters will need to stay on 24 hours a day. 

(Find out whether guppies and goldfish need heaters too!)

Do Outdoor Mollies Need A Heater?

We know that Mollies are a fish that prefer warm water, so now let’s discuss the options of attaining Mollies for ponds. 

The same rules apply to your outside ponds as they do to your indoor tanks. If you live in a hot climate, you will be fine to keep your Mollies outside as they are already used to the water temperature. However, in the winter, if it gets a bit cooler and dips below the recommended temperatures, it would be worthwhile to provide a heater so as to not cause any problems for the Mollies. Or alternatively, put them in a tank seasonally. 

If you are in a warmer climate with steady heat all year, you will likely need to focus on the pond environment and try to replicate what they would be used to in the wild.  

Now, if you live in a colder climate, Mollies may not be a good fish for you in terms of outdoor fishkeeping. Although some suggest that you may be able to get the Mollies acclimated, these are tropical fish and thrive in tropical climates, so you may come across some issues. 

The initial cost of setting up lamps for year-round heat may be costly, especially in the middle of winter when they probably will not have the same effect.  

So the answer is yes, outdoor Mollies would need heaters, but it comes with its own set of risks.  

(Have you ever wondered whether mollies can live with goldfish or guppies?)

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What Happens If The Water Gets Too Cold?

So by now, we are aware that Mollies do need heaters depending on where you live in the world. However, it is also worth knowing the risks if the water was to become too cold. This is especially necessary to know if you want to keep these tropical fish outside in cooler climates. 


If the temperature of the water Mollies lives in becomes too cold, it will actually stop them from growing! They will become slower and more lifeless until they eventually die. It would be like you or I living in minus conditions while wearing summer clothes; it’s going to have an effect! 


What with the last analogy, you can imagine that it would cause a lot of stress to slowly die from the cold! Well, fish will have the same response! Cold water will cause a tropical fish to be under an innate amount of stress and lead to a shorter life. 


Because fish are cold-blooded, they cannot regulate their own body temperatures. This means if the water is to become too cold, it will directly affect their immune systems. This would then likely lead to disease or parasites. It is important to remember that even short exposure to temperature change will greatly affect Mollies. 

(Do you know what the ideal pH level is for mollies and how to maintain it?)

What Happens If The Temperature Drops Suddenly?


As with the water getting too cold as previously identified, the Mollies will be more susceptible to disease and parasites! However, this sudden drop in temperature will only quicken the effects of the cold. This is a reason to think carefully before deciding to put Mollies outside in colder climates, even with attempted acclimating. 


A sudden drop in temperature may also affect the Mollies’ ability to reproduce. This is due to the amount of stress that the Mollies are under in cold water. In their natural habitats, Mollies would move to another area of water that is warmer; however, they do not have this option in a tank and rely on you to keep the temperature consistent. The stress will directly affect their growth and ability to reproduce as any energy will automatically filter to areas to keep the fish alive. 

orange molly | nice colors on this one | h080 | Flickr

What If The Water Gets Too Hot?

Although Mollies are tropical fish, there is such a thing as too hot!   


If the temperature of a tank or pond was to increase above the advised amount, it would cause issues for the good bacteria living in the fish habitat. This would then cause a build-up of ammonia and cause even more problems! 

As stated before, Mollies can survive in all sorts of conditions, but even they have their limits, so it is always a good idea to check your temperatures daily! 


Water over 32 degrees celsius will cause problems for the Mollies, as at this heat and higher, the level of oxygen in water decreases. The way in which fish breathe is to draw out air from the water. They will need more oxygen in warmer water as it also causes them to become more active, which means there is a great risk of suffocation occurring.  


Again, just as Mollies would feel in cold water, a high temperature can cause a great deal of stress for them! This is due to the rapid increase in heat as opposed to what they are used to. Again, they will not be able to move from warm spots like they would in nature.  

The lack of oxygen, as previously explained, will also cause the Mollies to become stressed, which may only add to their energetic states, thus making the situation worse. 


It is important to understand that even though these are versatile fish in terms of water environments, they still will face problems if the water temperature is not within safe limits. 

Mollies are tropical fish, so they are used to warmer climates, which needs to be accommodated for if you decide to keep these particular fish. 

We now know that the appropriate temperatures to keep them in are between 21-29.5 degrees celsius; anything over 32 degrees celsius will cause serious harm to them. 

It is also best not to keep these fish in outside ponds during winter in colder climates. Some suggest you may use heat lamps; however, this may not work, and you may risk killing them due to a sudden drop in temperature. If you wish to do so in this climate, it would be wise to just stick to keeping them outside during summer. You may still decide to try acclimating them at your own discretion. 

To conclude, yes, Mollies need heaters for their tanks to regulate the temperature to ensure they thrive! Supporting this will mean that they grow to their full size and color and live a happy life! 

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