All You Need to Know About: Water Temperature

All You Need to Know About: Water Temperature

What is Water Temperature?

In this case, water temperature is a physical property expressing how hot or cold something is.

Why is it important?

Fish and invertebrates are cold-blooded, so they are unable to regulate their temperature. The water temperature controls their metabolic rate, and too high or too low of a temperature can cause stress and lead to premature death. Higher water temperatures have lower oxygen concentrations robbing fish, corals, and other tank inhabitants of the oxygen they need to survive.

For many stony corals, the zooxanthellae that produce their unique and beautiful colors cannot tolerate temperatures that are too low (64 degrees or lower) or too high (above 84 degrees). The stress of temperatures that are either too high or too low will cause coral to expel their zooxanthellae, a process that is commonly known as coral bleaching. Temperature fluctuations of more than a degree or two can also stress out coral and cause them to turn brown.

Coral Bleaching
Coral bleaching occurs when water temperatures rise.

What is the Optimal Range?

Reef Aquariums: 75-78 degrees Fahrenheit

Corals love stability, so it is crucial to choose a temperature and prevent any fluctuations.

Saltwater Fish Only Aquariums: 75-82 degrees Fahrenheit

Although fish are more tolerant to temperature changes than corals, keeping a consistent temperature is still important.

Tools for Measuring Water Temperature

There are a few different options to choose from when measuring your reef tank’s temperature.  Some of these options are not as accurate as others though all are generally easy to use.

Digital Thermometer with Probe
Digital Thermometer with Probe

Digital Thermometer with Probe:

This is the most commonly used tool for measuring aquarium temperature today. To measure temperature, place the probe into the water, and the temperature displays on a digital readout. Place the probe anywhere to test the temperature in that specific area of the tank.

Hand-held Digital Thermometer:

A thermometer with a screen attached to the probe. Also known as the instant-read thermometer.

Digital & Analog Thermometers
Digital vs. Analog Thermometer

Traditional Analog Thermometer:

A glass cylinder or similar is submerged into the water and has a line that measures up to the aquarium’s temperature.  These can be tricky to use and hard to read accurately.

Sticker Strip:

This is a strip that attaches to the aquarium’s glass and reads the temperature.  Typically this device is one of the most inaccurate devices to measure temperature, and as such, we do not recommend using one.

How to Maintain Water Temperature

Many factors impact the temperature of your reef tank. Some of the most critical factors are ambient room temperature, local weather/climate, aquarium size (the larger the tank, the less the temperature will fluctuate), and equipment. Metal halide lighting or large return pumps, for example, can add a lot of heat back into the tank.

Submersible Heater
Submersible Heater

Submersible Heater:

Set the heater to your desired temperature, and it will automatically turn off and on as needed.  Some submersible heaters need to be connected to a controller and programmed to your desired temperature.  These are generally inexpensive and made from glass or titanium. Although more expensive, titanium heaters will not break or fail as easily as a glass heater.

Cooling Fan(s):


If only a modest decrease in temperature is needed, cooling fans blowing across the surface of the water may be enough to do the trick. Please keep in mind that blowing air across the water will increase water evaporation and necessitate more frequent water top-offs.


This external unit pulls water from the aquarium and cools it before returning the water to the aquarium.

And there it is!  Measuring and maintaining temperature is pretty simple and figuring out what is needed to maintain your reef tank temperature is pretty self-explanatory.  Almost all aquariums require a heater. However, if the temperature keeps fluctuating or getting too warm, a chiller will be necessary to keep your temperature stable.

Are you looking for products and accessories for your reef aquarium? Browse our catalog here or find a store near you. If you have any questions about salinity or other topics about reef aquariums, click here to contact us.

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