All animals can be divided up into two main groups, depending on their ability to regulate the internal heat of their bodies. These two groups are:
Homeotherms (warm-blooded creatures)
Poikilotherms (cold-blooded creatures).

Most animals belong to the latter group of poikilotherms, which includes fish, crustaceans,, amphibians reptiles. and most insects.

Poikilothermic animals have no internal means of controlling and regulating their internal body temperature.
The amount of heat produced through metabolism is low and there are limited mechanisms for the retention of this heat. As a result of this, the internal temperature of poikilothermic animals is always approximately that of the environment that it occupied (usually within 10ºC).

This means that poikilotherms can only regulate their internal body temperature by the external temperature of their surroundings The internal body temperature of a species has a direct effect on its metabolism.

It has been estimated that an organism's metabolism doubles with every 10ºC increase in temperature.
If a freshwater crayfish is in a cold environment, it becomes sluggish due to the reduction in its metabolism, which results in a slowing of its movements and a decrease in its feeding.Temperature also affects other water quality parameters. An increase in temperature decreases the amount of oxygen present in the water by decreasing the solubility. Stock also require more oxygen at higher temperatures, consume more food. and produce more wastes which, unless managed correctly, may lead to problems.