Most fish in the aquarium trade are tropical species. This isn’t surprising seeing as the most diverse and species-rich ecosystems are found in the tropics. For instance coral reefs hold an unbelievable number of different and unique species which are found nowhere else, and are all found in tropical ranges. 

As you can imagine, tropical oceans are found near the equator, this being the hottest part of the planet. Tropical oceans encircle the Earth and can be found in an equatorial band between the northern Tropic of Cancer (23.5° North latitude) and the southern Tropic of Capricorn (23.5° South latitude). 

The majority of the Indian Ocean lies within these parameters, as do the central portions of the Pacific and Atlantic Oceans. These oceans are warmed by the intense tropical sun, driving oceanographic and atmospheric processes which play a critical role in regulating climate and weather patterns.

In the tropics, as the sun is beating down strongly all year round, the sea temperature typically exceeds 68°F (20°C), and ranges from 66-82°F (19-30°C). 

These temperatures are very warm indeed, and so keeping animals from these regions requires the same conditions to be met. That is why a good quality aquarium heater and thermometer are essential components of any tropical reef tank, and indeed freshwater tank as well. 

Most species of tropical saltwater fish require a water temperature that falls within the range of 66-82°F, as this mimics their natural habitat. Be sure to check what range the individual species require though, as some may require the lower end of tropical temperatures, and some may require the higher end. Always only house species which require the same temperature range together. 

The surface sea temperature in the tropics remains pretty consistent throughout the year, as the intensity of the sunlight doesn’t vary. 

However as you would expect, the surface sea temperatures of temperate regions vary, often very wildly, with the different seasons, with higher temperature during the summer and lows during the winter months. In general temperate regions have a winter sea surface temperature of around 41-45°F (5-7°C), and a summer temperature generally of 56-63.5°F (13.3-17.5°C). These temperatures can vary depending on the conditions at the time. 

At the poles, the temperature is colder still. In the summer, depending on how close to sea ice, and intensity of the sun, the sea surface temperatures can range from 28.4-46.4°F (-2-8°C). In the winter the sea is often covered completely with ice, and temperatures stay at a cool 28.4°F. 

Keeping coldwater fish in aquariums is much less common than keeping tropical warmwater species. This is due to the vast amounts of species available from the tropics, and that they are much brighter colored, and so are more highly prized. However there are a large number of invertebrates such as echinoderms like starfish and sea urchins, and mollusks such as gastropods and chitons that can have very striking colorations and work very well in coldwater marine tanks. 

These are sea surface temperatures; the temperature also changes with depth. What happens is a very interesting quirk of oceanography, and is to do with the properties of water. 

Warm water is less dense than cold water, and so “floats” on the top. So at the tropics there is a layer of warm water which floats on the top of cold deep water. This layer is called the mixed layer and is about 100-200m deep. This is because the sun’s rays can only penetrate 100m, and the mixed layer extends to 200m due to wind and wave action mixing the sub-surface waters. What occurs next is a sudden drop in temperature over a small depth range, called the thermocline. Under the thermocline the water temperature remains pretty stable at 35.6°F (2°C). 

The thermocline is showcased differently at different latitudes, being much more pronounced at low latitude tropical regions as the surface waters are much warmer. In temperate mid latitude regions the thermocline is shallower and more pronounced during summer, but is less pronounced and gets deeper during winter, as storms churn up the oceans and mix the water to a greater depth.

At polar regions the water temperature is pretty consistent throughout the entire water column, and deep water can actually be a little warmer than surface waters sometimes!

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