Summary:  The consistency of the tank’s water temperature is more important than the actual temperature itself. The heater is just a device that keeps the temperature constant.  

To what temperature should I set my tank?

A fish tank’s temperature should be fit to whatever is living inside. This is common knowledge. With just a little bit of research online, you can easily find optimal temperatures for Guppy, Pleco, Discus, Shrimp, etc. However, most aquatic animals can survive in temperatures outside of this range. The numbers online merely reflect the most common temperature that species lives in, or possibly the temperature in their wild habitat. It is true that at those temperatures, the fish prosper and grow best.  

This is why most people purchase heaters online and set the temperature based on what their friend recommended and they think they are set. Nothing more to do. 

You can think this is the purpose of heater but, in truth, it is not. 

Then what is more important than the temperature of the water?

The thing more important than the temperature itself is...the consistency of the temperature.

Most tropical fish can survive between 59F to 39F.

Believe it or not, it’s true! However, it is based on the assumption that the temperature is stable. 

The temperature of the water is not as important as how consistent the temperature of the water is. Most household fish tanks hold much smaller amounts of water than a river or stream so it is more easily affected by the environmental temperature. 

You might have seen a graph like this. Large bodies of water change temperatures much slower than land. 

What does that have to do with the tank heater?

As stated before, having a consistent tank temperature is important. Depending on where the tank is located, the temperature around it will vary. Fishkeepers need to be aware of this and adjust the tank’s temperature accordingly. Also, with changes in seasons, you need to watch the temperature of the room the tank is in and if the room temperature changes, so must the tank’s. 

Take a look at the graph below.

It shows the current change in temperature in South Korea.

* Of course the temperature inside can be very different from the temperature outside. 

In a tank without a heater, the temperature will differ morning and night. If the difference exceeds 8C everyday, it can lead to stress for the fish and cause a weakened immune system thus making them more susceptible to disease. 

However, if you set the tank heater to 78 degrees, the change in temperature that occurs daily will range from 78 to 82 degrees. 

That is the main purpose of tank heaters, to keep a consistent temperature so the fish can be stress free and healthy. I am not saying that the temperature of the water does not matter at all. It is important too, however, a constant temperature is A LOT more important. 

How to correctly set the tank’s temperature

As stated above, the tank’s temperature setting must be changed based on the change in season. It is especially important in places with distinct seasons with extremely varying temperatures. Has your healthy fish been dying suddenly? I recommend logging the daily temperature changes to see how much it fluctuates. 

Happy fishkeeping! Thank you.