Articles Water quality

Aquarium thermometer 101 | Types & why you need one

April 3, 2016
aquarium thermometer

When setting up an aquarium and buying equipment, it can be easy to forget to pick up a thermometer. For beginning aquarists who do remember to get one, the different types of aquarium thermometers can be quite confusing. Which one works best for your aquarium?

Keep reading to find out why you need a thermometer in your aquarium, which types you can find and their pros and cons!

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 be able keep a close eye on your aquarium temperature yourself. You should always have a thermometer in your aquarium and check it at least every day.

Just a quick glance when you’re feeding the fish and after water changes can be enough to catch any problems early on and prevent fishkeeping disasters. There are two reasons for this!

  • Fish (and invertebrates) need stability. The right temperature is very important for most aquarium fish and inverts, but another crucial factor to keeping them healthy is a stable temperature. Aquarium temperature fluctuations, for example between day and night or when water temperature isn’t matched when doing water changes, can be very stressful. This in turn makes your fish vulnerable to diseases like fin rot.
  • Checking the temperature every day at different times allows you to spot any inconsistencies and help fix them. Having a good quality heater can also help prevent temperature fluctuations before they even happen.aquarium thermometer
  • Broken heaters and other incidents. Although a good heater and matching water temperature while doing water changes are usually enough to keep the water temperature stable, isolated incidents are always a possibility. Very hot or very cold water 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, check immediately whether your heater is still working. If it has malfunctioned, the tank may become too cold or dangerously hot.
  • Malfunctioning thermometer. Yes, even when you’ve already got a 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! I’ve actually had incidents where very old thermometers weren’t working properly or at all any more, which is obviously not ideal when you’re relying on them.

Types of aquarium thermometers

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

Classic floating/suction cup thermometer – These analog thermometers use the same mechanism as the one you may have hanging in your home. They float around the tank or are secured to the bottom or the walls of the aquarium (as pictured above) and you can tell the temperature from the red bar.

These thermometers are a great choice and should be able to give you an accurate reading of your aquarium temperature. Most feature a green bar that indicates a safe temperature zone. They’re also not expensive at all and will last for a very long time, although very old ones may eventually stop working. If you’re clumsy like me or keep fish that may damage a glass thermometer, getting a plastic one may be a good idea to avoid breakage; you don’t want to have to clean up all those tiny balls from your tank or the floor!aquarium thermometer

Digital aquarium thermometer – A digital aquarium thermometer (pictured above) usually looks like a small, rectangular device with a small LCD screen and sits on top of or next to the tank. A temperature probe is placed in the water to measure the temperature.

Digital thermometers are a good choice and usually very accurate. Most show both Fahrenheit and Celsius and unlike analog thermometers, which can be a bit difficult to read sometimes will allow you to see the temperature with an accuracy up to 0.1 °F/C. I’ve found them 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 once in a while.

Stick-on/liquid crystal thermometer – The simple stick-on thermometer is what most people associate with aquarium thermometers. It’s stuck to the side of the tank and displays the temperature with color.

These are a cheap option and fine when you don’t need to know the exact temperature down to 0.1 degree, but they’re not my favorite because they can be quite hard to read and most can’t be moved. They may also not be too accurate, although if you buy a high quality one it can still be a good option!

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

Cover photo: Work in progress by threefingers

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  • 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


      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.


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