Updated: Mar 25
One of the most important considerations when keeping ectothermic (cold-blooded) aquatic species is temperature regulation. Fluctuations in temperature, even in small increments, can have detrimental or even deadly effects.
Regulating the temperature in your aquarium isn’t just adding a heater, setting it to 78 degrees Fahrenheit and calling it good. Understanding the correlation between heat, water, and flow is important. The relationship between water and heat is complex and changes depending on the kind of water.
As ectotherms, fish and invertebrates rely on the ambient temperature surrounding them to regulate their body temperature. Fishes’ body temperature can affect their immune system, metabolism, color, and activity level.
Once you understand how heat works in an aquarium, and how fish respond to it, then you can choose which type of heater you want and carefully determine where to place it.
Heat and Water
When I first started researching this article, I began by looking up the insulative properties of water. I quickly found that insulation is not the right word. You may know that water holds heat more effectively than air. I thought this translated to water being an effective insulator. As it turns out, an insulator is a material which does not transfer heat (or electricity) effectively. That's when I discovered the term “specific heat”.
Specific heat is the amount of energy (heat) required to raise the temperature of 1 gram of a substance by 1 degree C. Water has the highest specific heat of any liquid. This is because of the high level of energy expelled from breaking hydrogen bonds and required to form hydrogen bonds as water heats up and cools down. Freshwater has a higher specific heat (4.18 Joules per gram Kelvin) than saltwater 3.99 J/(g K).
This relationship between heat and water is important to us as aquarists because temperature plays such an important role in the health of our tanks. With a high specific heat, water is very forgiving of mistakes, but it is also slow to react to changes.
Placing your heater in a low flow area can result in localized hot-spots and prevent heat from properly distributing throughout the aquarium. Changing the temperature of your entire aquarium takes several hours, due to water's high specific heat. This can be good as you don’t want to shock your aquarium's inhabitants. It is also fortuitous if your power goes out in a snowstorm. Water will lose heat slower than the air around it.
The advantages of the high specific heat of water doesn’t stop with the tank water. Fish are 70% to 80% water (Lakshmanan, P.T. et. al. 2013). The relationship between the water they live in, and the water inside their tissues is interconnected. Osmoregulation refers to a fish’s ability to control this relationship, and it is closely tied with metabolism and temperature regulation.
Fish are Ectotherms
Fish metabolism is heavily dependent upon the ambient external temperature because fish are ectotherms. An ectotherm is an animal or organism which relies on ambient external temperature to regulate, or assist in regulating, their internal temperature.
Fish metabolism is important because the rate at which your fish use energy (metabolism) directly affects their health and how much food they need to consume on a daily basis. Resting metabolic rate refers to the amount of energy required just to stay alive. Increasing the temperature from 50 degrees F to 68 degrees F will double the resting metabolic rate of fish. This means the fish will need to consume twice as much energy just to stay alive (Clarke & Johnston 1999).
As you can see, even small changes in temperature can mean drastic repercussions for your fish. Heating your aquarium is more than just setting your heater to 78 degrees. You need to make sure the temperature you choose is stable, uniform throughout the aquarium, and within the correct range for your chosen fish species.
Another factor of fish metabolism and thermoregulation which should be closely monitored is oxygen level.
The warmer the water gets, the less dissolved oxygen it can hold. Cooler water has a higher dissolved oxygen content than warmer water. Metabolism has a direct relationship with oxygen consumption. The higher your metabolic rate, the more oxygen you consume. If your fish live in water that is too warm, they will simultaneously need to consume more oxygen (higher metabolic rate) and have less oxygen to consume (lower dissolved oxygen content).
For this reason, it is vital to keep your tank at a stable, uniform, and most importantly, accurate temperature.
What feels cool to us is actually hot to fish. We, as endotherms, have an internal temperature of 98.6 degrees F. When we stick our fingers (~96 degrees F) in the tank the water may be at 77 degrees F, but it feels cold to us. That is the temperature differential of 96 degrees F and 77 degrees F at play. To an ectotherm (fish) it feels normal.
Our anthropocentric (human-focused) viewpoint may cause us to increase our aquarium temperature to what we find comfortable, but that could be detrimental to our fish. Because oxygen content decreases with higher temperatures, and oxygen consumption and metabolic rate increase, we want the temperature in our tanks to be on the low end of comfort for our fish. Obviously, we don’t want it too cold, but fish are much healthier when they don’t have to expend extra energy just to stay alive.
So what is the comfort zone for your fish? Fish habitats vary widely and cold-water vs. tropical species have different comfort ranges entirely. If you were to google “best temperature for tropical fish” you would get 49.2 million results all saying somewhere between 75 and 80 degrees F.
Would it surprise you to know that the Orinoco River, home to hundreds of tropical aquarium species like corydoras, cardinal tetras, German blue rams, and more has an average annual temperature of 67 degrees F. and rarely reaches above 69 degrees (Gomez 2020)? The Mekong River in southeast Asia can fluctuate between 74 and 89 degrees F (Owen 2021). The Negro River in Brazil averages 82.2 degrees F (Spectacular 2020). Lake Victoria in Africa stays at about 77.7 degrees F. (Xungang 1998), and the Great Barrier Reef maintains a temperature between 71 and 82 degrees F (AIMS 2022).
I’m not sure why the aquarium industry has agreed on 75 to 80 degrees F. as the “tropical temperature range”, but suffice it to say that natural waters can vary widely. I suggest doing some research to determine where your species are from and the temperature range they experience in their natural habitat.
You don’t want to set your aquarium temperature to the lowest temperatures recorded in that region, but 75 degrees F is not necessarily the minimum, and 80 is not the max. Discus, for example, will thrive in 82 degrees F. Also, consider that fish can move about within their natural habitat to find warmer or cooler waters, which is harder to do in your tank. It is important to note that the temperature of a massive river or an entire ocean changes over days or months, not hours like in the aquarium.
Finally, consider the fact that these species evolved within these natural temperature ranges, and are comfortable in them. Fish can adapt, but starting with well researched theory, as opposed to an arbitrary temperature range, is a great place to begin.
Heating The Aquarium
Now that we have an idea of how water, heat, and our fish work together, I’ll detail the best practices to regulate the temperature of your aquarium.
No Heater at all.
Just like with any other piece of equipment, heaters can fail. Simply failing to turn on is the best case scenario of a failing heater. There have been cases of heater thermostats malfunctioning and raising the temperature of the tank to above 100 degrees, effectively suffocating the inhabitants. While this is extremely rare, it can happen, and this is the main reason you want to buy a quality heater.
For those of us fortunate enough to live in a warm climate, you can avoid the tragedy of malfunctioning heaters by never installing one to begin with. Depending on the climate where you live, and your range of comfort, some aquarists believe maintaining the temperature of your home to match your desired aquarium temperature is a better option.
This may not work if you live in a climate where the temperature can reach 105 degrees F. and your air conditioner must be set on high 24/7, just to keep your tank below 80 degrees. Generally, humans like to exist in that 68 to 80 degree F range, just like tropical fish.
The failure rate of your home furnace or air conditioner is lower than that of most aquarium heaters. And, if your power goes out, you won't be able to use your aquarium heater anyway. Also, any temperature fluctuations in your house are mitigated by water’s high specific heat.
Think about your aquarium placement; forgoing a heater may not be a good idea if your tank is close to an air vent, or near a drafty window.
Types of Heaters
I use a heater in my tanks because I live in the Midwest. I like my house around 68 to 70 degrees F in the winter and 78 degrees F in the summer. For most of the summer, my heater rarely turns on, as the house temperature maintains the same temperature I have my heater set to. However, my heater is crucial to maintain that same temperature in the winter.
There are dozens of features and designs for aquarium heaters. Some are made of glass, others titanium. Some have temperature regulation built in, others have a separate controller or mobile application. Some heaters have LED displays, others have simple on/off lights. Regardless of the design and features, most heaters fall into one of two categories; thermostatic, or automatic. Within these two categories, there are different types of heaters categorized based on placement. These are: submersible, undergravel, filter, and inline.
Thermostatic heaters have a built in thermostat which allows for manual regulation of the temperature. These are by far the most popular type of heater, because you can tweak your temperature reading depending on placement and ambient household temperature.
Automatic heaters are not nearly as popular because they will regulate the temperature to a set degree. They do not have the capability to raise or lower the temperature beyond that programmed degree setting. Automatic heaters are most common for nano tank heaters, as there is not enough room on such a small heater to install a manual dial on the thermostat. This allows them to be used in very small tanks where temperature regulation is still necessary.
Where you decide to place your heater will affect the efficiency with which your heater regulates your aquarium’s water temperature. You want to choose a place free of clutter, with high flow. Most aquarists will opt to hide their filter as well. Here are some of your options for heater placement.
By far the most popular heater placement, submersible heaters can be placed almost anywhere in the main display tank or the sump. They even make modules that you can install the heater inside of to create an inline setup for maximum efficiency.
Submersible heaters usually allow for the top temperature dial portion to be out of the water, or completely submerged underwater. You can orient them vertically or horizontally depending on how it best fits into the aesthetic of your tank. I like my heater horizontal as it maximizes the area of the tank which is exposed to the heating element.
Regardless, make sure your heater is placed near good flow so the warmer water will effectively disperse.
This type of heater is not very popular because it is difficult to adjust, without a separate controller, due to it being under the gravel. It is also relatively inefficient, as it cannot reach the water column effectively. Additionally, the heating element is placed too close to the glass bottom of the tank for most aquarist’s comfort. There are plenty of other types and placements of heaters which allow you to hide the heater from view, without it being a hassle to adjust.
There are brands of canister filters that come with compartments designed to hold aquarium heaters. These are great because the dial is exposed allowing adjustment, the heater is hidden from view, and the flow through the filter allows for optimum exposure to the element for effective heater dispersal. Just be sure you unplug your heater when performing filter maintenance.
You can also place your heater in your sump if you want it hidden. There is usually optimal flow, and room for a heater. Be mindful of the compartment you place the heater in, as the water level in a sump can fluctuate during water changes and due to evaporation. This can expose the heating element, causing damage.
The final type of heater is an inline heater. This can be a heater designed only as inline, with input and output nozzles attached to a housing unit that contains a heating element. There are not many options available for inline heaters as they can be unreliable and difficult to replace if something goes wrong.
Another option for inline is to install a submersible heater into an inline heater module. This is a separate unit with input and output nozzles and a chamber to hold your stand-alone heater. These are the most efficient as the heater can be easily adjusted and replaced, and the water flows around the heating element, sufficiently exposing it, before entering the aquarium.
It is also worth mentioning that many heaters are made of glass. If you have a large aggressive fish in your tank, you may want to consider getting a heater guard. These are plastic cages that the heater sits in, which prevent it from bashing against the glass tank causing damage, or worse. Alternatively, you can consider installing your heater inline, or in the filter.
Efficient temperature regulation is important for your aquatic specimens. Use a handheld thermometer and measure the water temperature of your tank in many different areas (top, bottom, side closest to heater, side farthest). Also measure the temperature of the water in your filter. This will help determine if you need to change your heater placement, amount of flow, or change to a different type of heater altogether.
Don’t be afraid to change the temperature of your tank if you find out it is too high or low for the species you are keeping. Just make sure to change it no more than 1 degree Fahrenheit every few days. You want to give your fish plenty of time to acclimate. You can measure fish health in relation to temperature by monitoring their breathing, increased coloration, and increased appetite.
And always remember to record your results so you can recognize the patterns in your tank. This will allow you to make smart and deliberate decisions for your fishes’ health.
Australian Institute of Marine Science (2022). Table generated 17th February 2022 using Reef Monitoring. Long term Monitoring and Data Centre, AIMS. Viewed 17th February 2022.
Clarke, A., Johnston, N.M. (1999) “Scaling of metabolic rate with body mass and temperature in teleost fish”. Journal of Ecology. 68; 893-905.
Gómez, M. Fermín , Denevan, . William M. and Brunnschweiler, . Dieter (2020, April 2). Orinoco River. Encyclopedia Britannica. https://www.britannica.com/place/Orinoco-River
Lakshmanan, P.T., Mathew, S., Anandan, R., Asha, K.K., & Chatterjee, N.S. (2013) “Biochemical Analysis of Seafood”. Central Institute of Fisheries Technology. India.
Owen, L. , White, . Gilbert F. and Jacobs, . Jeffrey W. (2021, August 17). Mekong River. Encyclopedia Britannica. https://www.britannica.com/place/Mekong-River
"Spectacular confluence of the Negro and Solimões rivers, whose waters are separated for several miles". Visit Brazil. Retrieved 2020-01-25
XUNGANG YIN & SHARON E. NICHOLSON (1998) The water balance of Lake Victoria, Hydrological Sciences Journal, 43:5, 789-811