How do you read an aquarium thermometer?

Depending on the type of thermometer you chose, there are various ways to read their information. Digital output, such as from a digital, probe or infrared thermometer can be read on the display directly. Ensure that you have it in the correct Fahrenheit or Celsius setting. Traditional thermometers have a red line that will rise or fall depending on the water temperature. You may need to twist or tilt the thermometer in order to see this line appropriately.

Are stick on aquarium thermometers accurate?

You may note that there are no external, stick-on thermometers in our recommendations. That is because these thermometers are unreliable and do not provide any helpful information to fish owners. Yes, it may have been included in your aquarium kit, but a quality thermometer in direct contact with your tank water is a necessity.

Where do you put a thermometer in an aquarium?

As long as you have consistent water flow throughout your aquarium, it does not matter where you place your thermometer. If you have decorations or plants, you may need to ensure proper water flow with the addition of powerheads or outflows from other filters . If you are unable to move anything in your tank, you will need a thermometer at each end of your aquarium to ensure they do not differ too greatly. Your fish may congregate at one location or the other, depending on their temperature preference.

The 6 Best Aquarium Thermometers of 2023

No matter what type of aquarium you have, an accurate measure of temperature is critical. Being ectotherms, fish behavior and appetite is critically linked to their water temperature, so you may not be able to know what is normal for your fish if you don’t know your water temperature. There are several different types of aquarium thermometers available.

“Temperature is critical to fish health and disease progression,” Jessica Koppien-Fox, DVM, CertAqV (a special certification from the World Aquatic Veterinary Medical Association for specialists in aquatic veterinary medicine) told The Spruce Pets. “Knowing your tank’s temperature is one of the most important pieces of information that you can track and manipulate as a fish owner. Always test your water temperature and do not assume that your heater knows best.”

When it comes to selecting the best thermometer for any system, simpler is better, and our best overall pick, the Marine Deluxe Floating Thermometer with Suction Cup, exemplifies straightforward usefulness. It’s also affordable and easy to set up in your aquarium. But there are several other thermometers with different strengths that we also highly recommend.

Here are the best aquarium thermometers.

Our Top Picks

Best Overall:

Marina Floating Thermometer with Suction Cup at Amazon

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Best Laser:

Kizen Infrared Thermometer at Amazon

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Best Digital:

Zacro LCD Digital Aquarium Thermometer at Amazon

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Best Magner:

JW Aquarium SmartTemp Thermometer at Amazon

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Best Standing:

Marina Standing Thermometer at Amazon

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Best Probe:

ThermoPro Digital Instant Read Thermometer at Amazon

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In This Article

Our Picks
What To Look For
FAQs
Why Trust The Spruce

Marina Deluxe Floating Thermometer with Suction Cup

Amazon

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What We Like

Easy to use
Reliable temperature reading
Long-lasting
Color bands indicate safe zone for tropical fish

What We Don’t Like

Glass can break if improperly handled
Requires occasional cleaning

When it comes to choosing the best overall aquarium thermometer, bigger, fancier, and more expensive is not the best option. The Marina Deluxe Floating Thermometer with Suction cup is a bargain (and likely to be your least expensive piece of equipment), works instantly out of the box, and will last longer than your fish, if properly handled.

The deluxe model has a highlighted range of temperatures for a wider variety of goldfish and is easy to read. Unlike digital models, the old-school technology requires no calibration or fussing. You can use it in your tank, your sump or overflow for a reliable temperature reading whenever you need it.

The only real downside to the Marina Deluxe Floating Thermometer is that it’s made of thin glass and is easily broken if dropped or mishandled. However, this is a crucial component of analog thermometers, since the thin glass allows for the most accurate temperature reading.

Price at time of publish: $8

_{Material: Glass | Dimensions: 5.9 inches}

Kizen LaserPro LP300 Infrared Thermometer

Amazon

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What We Like

Useful for preventing cross-contamination across multiple tanks
Highly accurate and adjustable to different surfaces
No need to clean

What We Don’t Like

Requires batteries
Less reliable readings when water flow is inconsistent or low

Infrared thermometers are often used for cooking, but are becoming more common among aquarium hobbyists. Using the Kizen LaserPro LP300 Infrared Thermometer is simple: hold it about 14 inches away from a surface, point, press the trigger, then release. It will read the temperature of that surface instantly, with a high degree of accuracy.

In addition to providing instant results, the LP300 can read temperatures in Fahrenheit or Celsius, has a backlit LCD screen, and requires only two AAA batteries to operate. Plus, it never needs cleaning.

Aquatic veterinarians like laser thermometers like the Kizen LaserPro because it reduces cross contamination between systems. This makes it a great pick for anyone with multiple aquariums. You can take readings off the glass, which will be a similar temperature to the water, or measure the temperature by pointing directly down into the tank. Because laser thermometers measure the surface temperature at a very specific point, it’s possible that readings will be a degree or two off from the overall water temperature—especially if your tank is very tall (with multiple layers of water temperatures) or has low water circulation.

Price at time of publish: $24

_{Material: Plastic | Dimensions: 7 x 4.5 x 2 inches}

Amazon

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What We Like

Easy to read
Long cord for flexible installation
Reads Fahrenheit and Celsius

What We Don’t Like

No ability to recalibrate
Uses specialized LR44 battery (battery included)

Simple and easy to use, the Zacro LCD Digital Aquarium Thermometer includes an easy to read display which attaches to the exterior of your tank with a provided suction cup. It takes your water’s temperature using a submerged probe attached to the end of a 24-inch cord, providing enough flexibility to set the thermometer up in whatever way works best for your aquarium. (Be sure to only submerge the probe and not the display!)

Digital thermometers are easy to read and convenient, but are known to slowly lose calibration over time and produce readings that are a degree or two off. Affordable models like the Zacro LCD can’t be recalibrated, but should probably be replaced every few years.

Price at time of publish: $8

_{Material: Plastic | Dimensions: 2 x 1.5 inch, with 24.88-inch probe cable}

Walmart

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What We Like

Doesn’t require a suction cup
Easy to install
Measures Fahrenheit and Celsius
Green band indicates best temperature zone

What We Don’t Like

Small print
Small magnet easy to misplace

If the top or the bottom of your tank are hard to visualize or you’d rather take a temp reading right in the middle of the water column, then you need the JW Aquarium SmartTemp Thermometer. This thermometer attaches to the side of your aquarium using a magnet at the top of its body, allowing you to place it exactly where you choose. Plus, like our best overall pick, the SmartTemp includes a green band indicating the safe zone for your fish.

While we like the magnet attachment point, it would be very easy to misplace the small magnet used on your tank’s exterior.

Price at time of publish: $7

_{Material: Plastic | Dimensions: 4.5 x 0.5 inches}

The 7 Best Filtration Systems for Aquariums of 2022

Amazon

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What We Like

Easy to install
Captures temperature near substrate
Ballast maintains standing position

What We Don’t Like

Can be moved out of sight by water flow or fish

Standing thermometers are designed to sink to the bottom of your aquarium and rest gently on the substrate. The Marine Standing Thermometer is inexpensive, easy to use, and gives reliable results.

The biggest issue with standing thermometers is that they can be moved around by flow within the tank or the tank’s inhabitants, making the temperature difficult to read. If you have a very deep tank, your surface and bottom temperatures may vary, making it a good idea to maintain both a floating and standing thermometer for the most accurate readings.

Price at time of publish: $3

_{Material: Glass | Dimensions: 4 inches}

ThermoPro Digital Instant Read Thermometer

Amazon

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What We Like

Provides instant readings
Measures Fahrenheit or Celsius
Backlit LCD
Multiple ways to store
Kitchen multi-tasker

What We Don’t Like

Cannot be left in aquarium
Thermometer body is not waterproof

Another crossover from kitchen appliances is the ThermoPro Digital Instant Read Thermometer. This thermometer has a metal probe designed to measure the temperature of meats and other kitchen applications, but it will also work for your aquarium (but make sure to clean off the cooking grease!).

Our favorite probe thermometer is the ThermPro TP03 Digital Instant Read, which is quick, accurate, and flexible, with a probe that folds out at any angle and back into the thermometer’s body for storage. It’s got a backlit LCD, and only requires a single AAA battery to operate. Storing the ThermPro is particularly easy, since it’s both magnetic (so you can slap it on the side of a fridge) and has a loop at the top, if you’d prefer to hang it somewhere.

It’s even available in your choice of black or orange-red colors.

Price at time of publish: $14

_{Material: Plastic, metal probe | Dimensions: 6 x1 inches}

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Final Verdict

The best aquarium thermometer is simple, reliable, and easy to use, all qualities exemplified by our favorite, the Marina Deluxe Floating Thermometer with Suction Cup. It’s a great choice for any tank type and will last you for years, like mine has! If you have difficulty reading traditional thermometers, or have multiple aquariums in your home, then check out the Kizen LaserPro LP220 Infrared Thermometer, our favorite laser thermometer, which allows you to take quick readings from outside of the water.

Installing

No matter what type of thermometer you choose, installation is usually simple and straightforward. Suction cup and magnetic models should be mounted in an easy to read location. Digital thermometers need to be hooked up to a probe which is then placed in the aquarium, usually in the corner furthest from your filtration.

Calibrating/Testing

Analog thermometers no longer use mercury, but are equally as accurate with mercury substitutes. Glass thermometers are fragile, but can achieve highly accurate readings because they are permeable to water temperature. They may need to be cleaned from time to time as algae builds up along their outer surface.

Digital models may start to trend upwards or downwards a few degrees as they are used. If you suspect your thermometer may be off, it is always easy to add another one, usually of a different type, to see if it corroborates your temperature or disagrees. If you have two conflicting thermometers, you will need a third to tell you which is correct. Thankfully, our top pick is cheap and easy to add!

How To Read

Digital outputs are very easy to read but be sure they are in the correct Fahrenheit or Celsius setting. For traditional thermometers, you will need to twist the thermometer until you see a red line along the long axis of the thermometer in between the number ranges. The top of the red line will correlate with the temperature in the tank. You may need reading glasses and bright light to ensure you can properly read the temperature. Try to install your thermometer in the tank where you can see the red line without having to move the thermometer.

Placement in Tank

For most smaller tanks under 20 gallons, unless you have significant mass of decorations, it does not matter where you place your thermometer. For larger systems over 20 gallons, place your thermometer in the corner of your aquarium furthest away from your filter outflow. If it is evenly placed between left and right, you may choose whatever area is easier to access.

FAQ

How do you read an aquarium thermometer?

Depending on the type of thermometer you chose, there are various ways to read their information. Digital output, such as from a digital, probe or infrared thermometer can be read on the display directly. Ensure that you have it in the correct Fahrenheit or Celsius setting. Traditional thermometers have a red line that will rise or fall depending on the water temperature. You may need to twist or tilt the thermometer in order to see this line appropriately.
Are stick on aquarium thermometers accurate?

You may note that there are no external, stick-on thermometers in our recommendations. That is because these thermometers are unreliable and do not provide any helpful information to fish owners. Yes, it may have been included in your aquarium kit, but a quality thermometer in direct contact with your tank water is a necessity.
Where do you put a thermometer in an aquarium?

As long as you have consistent water flow throughout your aquarium, it does not matter where you place your thermometer. If you have decorations or plants, you may need to ensure proper water flow with the addition of powerheads or outflows from other filters. If you are unable to move anything in your tank, you will need a thermometer at each end of your aquarium to ensure they do not differ too greatly. Your fish may congregate at one location or the other, depending on their temperature preference.

As an aquatic veterinarian, Dr. Jessie Sanders, DVM, CertAqV, knows how important your water is to your fish health. She has personally used all of these thermometer types before and is well acquainted with their best usages. The simple thermometer she chose for our top pick is a slightly upgraded model from the one she has used in her own aquarium for years.

With 10 years of practice as an aquatic veterinarian, Dr. Sanders has hands-on experience with a wide range of aquarium products, including at-home tests of fish tank equipment. She is the chief veterinarian at Aquatic Aquatic Veterinary Services and is a certified aquatic veterinarian through the World Aquatic Veterinary Medical Association.

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