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Aquarium thermometer 101 | Types & why you need one

Last Updated June 14, 2020
aquarium thermometer

When setting up an aquarium and buying the right equipment, it can be easy to forget to pick up a thermometer. For anyone who has never bought one, the differences between analog and digital thermometers can usually be quite confusing. It is important to know what kind of thermometer will be the most accurate and work the best for your specific aquarium.

Keep reading to find out why you need to use a thermometer, what types of aquarium thermometers are available, and the pros and cons of each!

Why do you need an aquarium thermometer?

While most aquarists are aware they need a heater for their tropical aquarium, not everyone knows just how important it is to have an accurate aquarium thermometer that is easy to use and read. You should always use an aquarium thermometer no matter what kind of tank you have and check it at least once every day, if not more.

Checking your thermometer when you’re feeding your fish or after water changes is an easy way to catch any temperature problems early on and can help prevent a complete disaster in your tank. There are three main reasons why you always want to make sure your thermometer is working its best in your aquarium:

Fish and invertebrates need water temperature stability. The right temperature is very important for most aquarium fish and invertebrates to sustain homeostasis. Even more importantly, a stable water temperature will make the difference between a healthy tank and an aquarium that crashes overnight.

Water temperature fluctuations can be very stressful for all aquarium inhabitants; temperature swings may be caused by changing room temperature throughout the day or failure to match new water temperature to the temperature of the aquarium when performing water changes. Any drastic change in water temperature can stress out your fish, making them more vulnerable to diseases like fin rot. If these temperature changes are quick enough and not addressed immediately, it is possible to quickly kill off anything currently living in the tank.

Checking the water temperature every day by checking the thermometer at different times allows you to spot any inconsistencies and help fix them. Having a good quality and accurate heater can also help prevent temperature fluctuations in your tank before they even happen. While it may seem like the best idea to go with the cheapest option when picking up an aquarium heater, heaters often break and it’s not uncommon to hear of someone losing all their fish because of this.

When performing water changes, it is crucial to match the water temperatures between the new water and the water already in the tank. Always use a thermometer to make sure the temperatures match before completing the water change to save your fish and invertebrates from becoming too stressed out and having your heater overwork.

aquarium thermometer

Broken heaters and other incidents. Although having an accurate heater and making sure to match new water temperature while doing water changes is usually enough to keep the temperature of the tank stable, isolated incidents are always a possibility. Water that quickly turns very hot or very cold can quickly kill your fish, but a working aquarium thermometer that’s checked regularly can help you catch any temperature differences early on.

If the temperature in your tank is suddenly drastically different, immediately check whether your heater is still working. If it has malfunctioned, the temperature of the tank may become too cold or dangerously hot. Another way to prevent this is by doing regular maintenance on your heater; every six months or so, you will want to remove it from your tank and clean it with aquarium water, making sure there is nothing outwardly wrong with it.

Malfunctioning thermometer. Yes, even when you already have an aquarium thermometer in your tank it may be a good idea to occasionally use a second one to check whether the first one is still actually working and giving an accurate reading! I’ve actually personally experienced when a very old thermometer gave the wrong water temperature reading, which is obviously not ideal when you’re relying on it every day for the success of your tank.

What is the best and most accurate aquarium thermometer for your fish tank?

If you want to keep a close eye on your temperature but aren’t sure what the pros and cons of different aquarium thermometers are, buying one can be a bit confusing. There are three different types of thermometers that all have their advantages and drawbacks but are pretty easy to read:

Classic floating/suction cup thermometer. These analog aquarium thermometers use the same mechanism as the ones you may have hanging in or outside of your home. They either float around the tank or are secured to the bottom or the walls of the aquarium (as pictured above) using a suction cup. You can then read the temperature from the red bar after allowing a few minutes of being submerged.

These aquarium thermometers are a great choice and should be able to give you a pretty accurate and easy reading of your temperature. Most feature a shaded green range that indicates a safe temperature zone for a typical tropical fish tank. These thermometers are also not very expensive and will usually remain accurate for a very long time, although very old thermometers may eventually stop working and become uncalibrated.

However, the suction cups on these thermometers also have the tendency to stop working after a while and may refuse to stay sealed to the side of your tank. A good way to prevent this from happening so quickly is by completely cleaning the side of the tank and making sure there is no algae or other slim that could make the suction cup less effective. If you find that your suction cup has stopped sticking to your tank, you can either buy a new one or get a new aquarium thermometer entirely as they are very affordable.

If you’re clumsy like me or keep fish that may damage a glass thermometer, getting a plastic one or a digital thermometer may be a good idea to avoid breakage; you don’t want to have to clean up a thermometer spill in your tank!

aquarium thermometer

Digital thermometers. digital aquarium thermometer (pictured above) usually looks like a small, rectangular device with a small LCD screen that gives a very accurate digital reading. A digital thermometer sits on top of or next to the tank while a temperature probe is placed into the water to read the temperature.

Digital thermometers are a good choice and usually one of the most accurate thermometer choices available. Most show both Fahrenheit and Celsius on the digital display and unlike analog thermometers, which can be a bit difficult to read, sometimes will allow you to read the temperature with an accuracy up to +/- 0.1 degrees. I’ve found digital thermometers to be long-lasting and capable of surviving being dropped. The only downside is that they can be a bit more expensive than analog thermometers and need replacement batteries every once in a while.

Stick-on/liquid crystal thermometer. The simple stick-on thermometer is what most people associate with the typical fish tank thermometer. These thermometers stick to the outside of the aquarium and display the temperature with changing colors.

These thermometers are a cheap option and not very recommended even if you don’t need to know the exact temperature reading down to 0.1 degrees. These thermometers can also be quite hard to read and are actually more likely to read an average of the room temperature and of your tank temperature than just the tank temperature alone. Stick-on/liquid crystal thermometers also can’t be moved once position on the outside of your tank, so if you want to reuse one or transfer it to another tank, these thermometers are not the best option for the long run.

While this kind of thermometer may not be the most accurate, it can still be a good option if you get a high-quality one and have a pretty consistent room temperature throughout the year.

How warm should a fish tank be?

The water temperature of your tank entirely depends on the type of fish you plan on keeping in your aquarium. There are also a few cases where you would need to closely monitor water temperature to help with diseases and/or illnesses. An accurate thermometer will help give your fish and invertebrates the appropriate temperature they need to survive:

Tropical fish tank. Typical tropical fish prefer a temperature range between 72°F and 82°F (22°C and 28°C). Usually, classic floating/suction cup thermometers will have this temperature range shaded in so that it is easy to read. Even more important than keeping your temperature within this range is actually keeping it at a stable value. Many aquarium keepers maintain their tank at an ideal temperature of 78°F (25.5°C). In this kind of aquarium setup, you could get tetras, plecos, cichlids, angelfish, guppies, gouramis, and many other types of tropical species.

Coldwater tank. A coldwater aquarium is usually best for fish that prefer a lower temperature range that is outside of the tropical zone that many other fish prefer. Either an analog or digital aquarium thermometer will be able to read these lower temperatures, though it may be easier to quickly read and understand a digital temperature reading: you won’t get confused by the shaded-in optimal tropical temperature range and it makes it easy to document accurate temperature fluctuations as they naturally happen from day-to-day.

Coldwater species are pretty forgiving and most have been bred to actually survive in an aquarium with higher temperatures, but you should always do research beforehand to make sure you understand their optimal range and use an accurate thermometer to measure and maintain it. Some popular easy fish choices that you can get for a coldwater aquarium or outdoor pond are white cloud minnows (60-72°F/15-22°C), goldfish (62-71°F/17-22°C), danios (68-72°F/20-22°C), and many other species that you can read about here.

Illness. There may come a time when you need to raise the temperature in your aquarium to help sick fish recover from illnesses, such as ich. During this time, it is very important to use an accurate thermometer; you will need to increase the temperature in even increments every few hours until you reach the upper 80s. Because you need to monitor the temperature so closely, a digital aquarium thermometer is best, but an analog thermometer will work just as well as long as you get the closest temperature reading as possible.

Other exceptions. There are some aquarium fish that have very specific temperature demands and that are usually kept by only the most experienced aquarists. Almost always, these aquarium keepers will use the best and most accurate digital thermometers and heaters as any fluctuation could cause the tank to crash. One species that is particularly challenging is the discus (Symphysodon sp.). These fish actually prefer an aquarium temperature around the mid-80s, and may not eat if temperatures are too low or too high.

Conclusion

If you have any more questions about aquarium thermometers in general, keeping your aquarium at the right temperature, or if you want to share your experiences with analog versus digital thermometers, be sure to leave a comment below. Happy fishkeeping!

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5 Comments

  • ReplyKim GoodwinSeptember 24, 2018 at 6:47 am

    Hi, Mari. I used to check the temperature of the water every day in the aquarium. Even though red algae develops in my aquaria. I couldn’t find the reason… Can you suggest me some ideas to keep my fishes safe?

    • ReplyMariSeptember 25, 2018 at 2:35 pm

      Hey,

      Sorry to hear you’re having issues with the algae. Do you grow live plants in your tank? Are you dosing nutrients? Algae are generally caused by an imbalance somewhere, whether it be too much light, too little Co2 or too few nutrients.

  • ReplyJudyApril 4, 2016 at 9:57 pm

    Great post as usual! Can’t stress the “good quality heater” part enough. I’m new to aquariums and made the mistake of purchasing a cheap heater on Amazon. Ended up developing red algae that killed some fish and apple snails. Found out from the pet store that rays from cheap heaters will cause the red algae. Wish I would’ve known…

    • ReplyMariApril 8, 2016 at 10:14 pm

      Wow, never heard of that! I’ve used cheaper heaters before but luckily they worked great and I never had any problems. It’s just so important to prevent malfunction or catch it early if it does happen.

  • ReplyGregg MartinApril 3, 2016 at 4:16 pm

    Great information. In my experience the stick on thermometers can be tricky as where to place. They are likely to be influenced by outside influences such as a desk lamp, sunlight and other heat sources. Nonetheless they cam still be part of a two thermometer redundancy system.

    Gregg

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