Frequently asked questions on temperature


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Ever wondered why different temperatures are recommended for different fish? Matt Clarke answers some of the more technical questions relating to temperature and the way it affects your fish.

Are fish cold-blooded?

Sort of. Fish are poikilothermic, so when the water temperature changes so does their body temperature. A few fish, including some catfishes, can change their body temperature, and some tunas and sharks maintain a body temperature a few degrees centigrade above the ambient water temperature.

In what ways does the water temperature affect fish?

Temperature affects a number of physiological processes. For example, pond fish become inactive during the winter months because their metabolic rate slows in lower temperatures.

At higher temperatures, water is capable of holding less dissolved oxygen. As fish have a greater demand for oxygen at higher temperatures, the end result is breathing difficulties or oxygen starvation.

Rapid temperature changes effect the bodily functions of fish and can be harmful, particularly to those adapted to specific temperatures, such as cool, well-oxygenated rivers.

How sensitive are fish to fluctuations in temperature?

Fish can detect small changes in temperature - some as small as 0.03C. Some fish succumb to whitespot and other stress-related diseases, while others tolerate change fairly well, as long as it's gradual.

Marines, fishes from large stable ecosystems like big lakes, and those adapted to cool water (or very warm water) are generally least tolerant of fluctuations.

As fish species have evolved to live at different water temperatures, each has a different tolerance. The maximum tolerable temperature is called the Upper Incipient Lethal Temperature (UILT) and the minimum is called the Lower Incipient Lethal Temperature (LILT). However, some fish are "eurythermal" and can tolerate a wide range of temperatures.

Goldfish, for example, can live in water from 1-40C/34-104F providing they are properly acclimatised. Other species, are "stenothermal" and have a narrower temperature band.

Kept at the wrong temperatures, fish will become stressed and may fall ill or die. If the temperature is too close to the LILT the fish suffer from hypothermia, and if it's too close to the UILT they suffer from hyperthermia.

In many cases the LILT and UILT change as your fish mature, which is why fry and young fish are much more sensitive to changes.

Can any fish tolerate extreme temperatures?

A few fish are capable of withstanding very high water temperatures. Some killifishes from the Death Valley area live in water up to 45C/113F, and some tilapiine cichlids can safely swim through hot springs of 70C/158F. Few can survive long periods in water over 45C, as their proteins begin to denature above this level.

Lots of fish can tolerate very cold conditions, too. At both poles, fish have developed special biological antifreezes, based on glycoproteins, which stop them freezing when the seawater drops below zero.

Interestingly, the notothenioids at the Antarctic and the cods at the Arctic have evolved almost identical antifreezes independently. Many polar fishes are so well adapted for life in the freezer that they die at temperatures of just 5C/41F.

What if my fish start overheating?

Because fish need more oxygen when they are warm (and warm water holds less oxygen), they usually succumb to oxygen starvation. Boost oxygen levels by adding extra aeration and bring the temperature down by placing bottles of cold water in the tank, or conducting a small water changes and topping up with cold dechlorinated tapwater.

What "tropical" fish can I keep in coldwater?

Although some tropical fish, like certain Corydoras, can tolerate slightly lower temperatures than normal if slowly acclimatised, the process could be stressful, so experts don't recommend it.

We could probably survive fairly well sleeping outdoors without any bedding, but we'd probably find the experience rather uncomfortable and would soon catch a cold or flu. You should try to keep your fish under conditions they would experience in the wild.

Should I top up with water the same temperature?

You should ensure that replacement water is close in temperature to the water in the tank. However, topping up with cold water can trigger many fish species into spawning if the fish have been conditioned.

As sea temperatures don't fluctuate as much, replacement saltwater should be the same temperature as that in the tank.

How long does it take new purchases to adjust?

While it can take fish a long time to truly acclimatise to a new temperature (or different chemistry), research shows that floating fish bags on the water surface for a few minutes prior to unpacking increases their resistance to thermal stress. A polystyrene fish box or picnic hamper is useful for keeping new purchases warm (or cool during summer) on your way back from the shop.

This is an item from the Practical Fishkeeping website's archives. It may not be reproduced without written permission.