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  • Writer's pictureStewart Kessel

What Temperature should my Aquarium be?

Updated: 6 days ago

Fish unwell as fish tank water too hot

What is the optimal temperature for a freshwater fish tank?

Every fish keeper must ensure the proper environment to guarantee the health and happiness of their aquarium inhabitants. The temperature of the aquarium water plays a fundamental role in maintaining such an environment.


Before introducing new fish or other livestock, you must research to ensure you provide them with the optimum water chemistry and physical environment. To better understand aquarium water chemistry, we recommend reading our article, 'A Beginner's Guide to Water Chemistry. '

For your livestock to remain healthy, the water quality must be excellent, and its parameters must be as close as possible to the inhabitant's natural environment.

In addition to water quality, hardness, and the water's pH (acidity, alkalinity), temperature is one of the most important factors to research. You must establish what temperature to maintain in the aquarium to mirror the inhabitant's natural environment.


Aquarists often categorise freshwater aquarium inhabitants as Coldwater, Temperate, or Tropical. Remember that fish are poikilothermic, which means the surrounding water regulates their body temperature and metabolism. Therefore, it's crucial to maintain and monitor the aquarium's water temperature to align with your livestock's natural needs. It is also important to point out that you should always avoid any sudden or prolonged changes in the measured temperature.


Poorly maintained water temperatures will adversely affect your fish's health, as they need specific conditions to thrive. Temperatures too low or high, or sudden temperature changes, will cause stress and health issues and allow diseases to take hold.


Water temperature also affects oxygen levels and other chemical balances. Higher temperatures reduce the dissolved oxygen level, which is something to consider when warmer weather adversely influences your aquarium water temperature. See our article on how to maintain your aquarium temperature during hot weather.

What temperature should I keep my aquarium water? 

The correct temperature for your freshwater aquarium water depends entirely on the fish you plan to keep. As suggested above, the conditions in your aquarium, from water chemistry to the physical environment to the temperature, depend entirely on the specific fish and other livestock you intend to keep.

The correct question is, what temperature does your fish require?

Coldwater aquarium fish are typically kept between 15 and 24°C (60-75°F). Temperate fish normally have natural environments between 18 and 23°C (64-73°F), while most tropical freshwater fish have natural environments between 24 and 27°C (75-80°F).


Whilst the classification of Coldwater, Temperate, and Tropical can indicate the appropriate temperature conditions, it is better to research the specific temperature of the species you plan to keep. For example, the temperature must be at the range's cooler or hotter end for some tropical species. Discus fish, for example, need to be kept at the highest end and thrive better at slightly higher temperatures, 26 to 30°C (82-86°F).


My advice, as before, is to do the background research and establish the specific needs of the species you are interested in keeping. Take care when selecting different compatible fish species to keep in your aquarium together. They should also be happy to co-exist in the same temperature range.


Examples of commonly kept freshwater species and their natural temperature range:


Pterophyllum scalare 

24°C - 30°C

Neon Tetra

Paracheirodon innesi

20°C - 26°C


Poecilia reticulata 

20°C - 28°C


Xiphophorus maculatus

20°C - 25°C

Tiger Barb

Puntigrus anchisporus

21°C - 26°C

Paradise fish

Macropodus opercularis

16°C - 26°C


Symphysodon discus 

26°C - 30°C

Peppered Cory

Corydoras paleatus

18°C - 23°C

Dwarf Gourami 

Trichogaster lalius

25°C - 28°C

Ember Tetra   

Hyphessobrycon amandae

24°C - 28°C

Penguin Tetra

Thayeria boehlkei   

22°C - 28°C

Harlequin Rasbora

Trigonostigma heteromorpha 

22°C - 26°C

Source: FishBase

What equipment do I need to maintain the correct temperature in my fish tank?


An essential item for monitoring aquarium temperature, thermometers come in various forms, from simple glass thermometers to stick-on labels that change colour to indicate the temperature to electronic probes with digital readouts. I advise buying a good quality, accurate thermometer that is easy to read.

Glass Thermometer

  • Good accuracy

  • Simple to read

  • Low cost

Digital Stick-on Thermometer

  • Small and discrete

  • Viewable at a distance

  • Colour change indicates temperature

  • Accuracy can degrade with time

External Digital Thermometer

  • Clear digital readout

  • Viewable at a distance

  • Good accuracy

Digital Probe Thermometer

  • Clear digital readout

  • Good accuracy

  • High/low temperature alarm

  • Small decrete probe with 1m cable

  • Display remotely located up 1 metre


Unless you live in a tropical environment or purposely space-heat the room, you will require an aquarium heater and thermostat to maintain the minimum temperature of your aquarium.


Even with cold-water or temperate setups, you may require a heater thermostat, even though the temperature will be lower. When selecting a heater thermostat for this application, ensure it has a suitable temperature range.


Equipment to heat your aquarium comes in various forms. Which type you choose depends on your specific needs. The most common is a fully submersible heater thermostat unit. These sit inside your aquarium, typically in a back corner. It is best to check with the manufacturer’s instructions for correct placement and orientation.

Submersible Heater Thermostat

  • Lower cost option

  • Good efficiency

  • Numerous power ratings available

If you have an external filter, inline heaters take advantage. They connect in line (hence the name) with the return pipework, re-heating the water as it passes through. These are a good option if the filter has a sufficient water flow rate and offers a clutter-free view in the aquarium.

Inline Heater Thermostat

  • Good efficiency (check water flow)

  • Clutter-free aquarium

  • Ensure pipework is secure

A similar approach to inline heaters is to purchase an external filter with a built-in heater. These, like the inline heaters, have the advantage of minimising the amount of equipment in the aquarium, although perhaps they need a bit more management when completing regular filter maintenance. My favourite filter heater combination has an easily replaceable heater unit and, therefore, offers excellent performance and easy repairs should they be needed.

External Heater/Filter

  • Good efficiency (check water flow)

  • Clutter-free aquarium

  • Simplified & compact set-up



These devices are generally not required for most setups. Still, chiller and 'cooler' units are available for instances where the environment does not enable a sufficiently low temperature to be maintained.

Chiller units are refrigeration units that use a heat exchange process, like your fridge or freezer. They can cool water as it is pumped through them.

Chiller Unit

  • Generally not required

  • Useful in hot environments

  • Specialised refrigeration unit

  • Very effective when required

Cooler units generally are fans that blow air across the water's surface. Ripples on the water’s surface create a larger surface area, allowing more heat exchange through evaporation.

Cooling Fan Unit

  • Easy Installation

  • Requires open top aquarium

  • Helps during warm weather


Frequently Asked Questions about Aquarium Temperature

What power (wattage) of heater do I need for my aquarium?

A general rule is that your heater needs to be rated at 1 W per litre of water in your aquarium. So, for a 200L aquarium, you would need an aquarium heater rated at 200 Watts. Don’t worry if the values are not exact, but it is better to have the heater value on the slightly higher side.


Based on the above calculation, you should avoid having a heater with a too-low rating, as it will need to heat for a more extended period and may cause a lag in reheating an aquarium. Conversely, choosing a heater rated at a too-high wattage can cause fluctuations as the heater switches in and out for short periods.


Once set, do I need to adjust the temperature of my aquarium?

No. Although the temperature in the natural environment may fluctuate during the seasons, there is usually no need to mimic this in your aquarium.


Aquarists looking to promote mating and breeding behaviour in their aquarium fish may look to fluctuate the temperature to trigger a breeding cycle. However, this is for the more advanced fish keeper who will have thoroughly researched the fish species they are keeping and typically will have separate dedicated breeding facilities.


Do I need to check my aquarium temperature regularly?

Yes. It is advisable to check your aquarium temperature daily. Set a routine to check at least once daily, but with a convenient thermometer permanently situated in the aquarium, it is easy to check it every time you admire your aquarium inhabitants.


Should I switch off the aquarium heater during hot weather?

No. Aquarium heaters will have a thermostat, either built-in or as a separate unit. These regulate the temperature and will not switch on the heater if the aquarium water exceeds the pre-set limit.


During extended spells of extreme, hot weather, where ambient room temperatures might mean the aquarium water is above the ideal temperature, you still should not unplug your aquarium heater/thermostat. It will not be heating your aquarium and will be required as soon as the aquarium temperature falls below the pre-set limit.


Does that aquarium water temperature have to be exact?

No. As long as the recorded temperature over time remains within the optimum range for the species of fish you are keeping, then this is okay.


Is the location of my aquarium important when considering aquarium temperature?

Yes. It is always advisable to locate your aquariums away from direct heat and drafts. Keeping an aquarium next to a heat source, such as a house central heating radiator, or near an external thoroughfare could cause unnecessary temperature fluctuations.


Author: Stewart Kessel CChem, MRSC



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