Keeping a Fish Tank Warm Without a Heater

In Aquarium Heatersby PaulK

Let’s say your power suddenly went out or your aquarium heater is malfunctioning and you don’t have a spare one. How do you keep the aquarium warm for your tropical fish?

If it’s for a short time, you can use direct sunlight, but be careful not to overheat the tank water. It’s a good idea to have a thermometer to monitor the water temperature. You don’t want to do that for days on end, because algae will start to grow insanely fast.

Another thing you can do is keep the aquarium lights on longer. Keeping the aquarium lights on longer is generally not the best thing either since more light creates more algae if you have phosphate present.

You can also just add some hot water from the tap. Obviously, you’ll have to siphon some of the existing water out to make room. Don’t forget to treat the tap water with conditioner and then monitor the new temperature of the water.


Most of the heat of the water will escape at the surface of the water, so to prevent that, apply some saran wrap over the top of the tank. That will keep the temperature of the water longer than if you just left it.

Some people wrap towels around the tank, but then you can’t really see the fish can you?


Heaters can be dangerous if the water level gets too low.
Heaters malfunction and can overheat the water frying your fish.
Heaters can be difficult to calibrate correctly.

The case can be made that you don’t even need a heater for your aquarium. Many species can survive quite well in water temperatures as low as in the 60s Fahrenheit. Consider that in the wild, water temperatures are not consistent. There can be a range of 15-20 degrees from morning to afternoon.

Now most of us keep our homes heated in the winter probably in the range of 68 to 75 degrees. If the room temperature is that, then the aquarium water will be about the same. You should get an aquarium thermometer to monitor that.

Summer months might be an issue if you live in a hot environment. You probably use some air conditioning that may cool the water down quite a bit. Again, this is where an aquarium thermometer is necessary.

Certain fish species like guppies or goldfish naturally survive in colder temperatures and you probably shouldn’t have a heater. They are very hardy fish. Some species, like discus or angelfish, need warmer temperature so if you have them you’re going to need to keep the water in the 75 – 80-degree range.

Depending on the species of fish, you may not have to worry about using an aquarium heater at all.

Spread the love