Temperature
While mammals have a regulated and practically stable internal temperature, that of fish varies according to the temperature around them, e.g. they are poikilothermic animals (“poikilo”=”varied” , “thermia”=”heat”). Each fish species has an optimal temperature for growth, determined by the optimal temperature of enzyme activity. Since enzymes control the rate of every biological activity including immune responses, maturation, feeding, digestion, and energy production, the optimal temperature for enzyme activity will be close to the optimal temperature for growth.
The magnitude of the temperature range that species can tolerate differs between species. The majority of salmonids, for example, have temperature optima within the range 12-17°C, whereas many cyprinids have optima above 20°C. In addition, there may be ontogenetic changes in temperature optima for a given species, with larvae and juveniles often having a higher optimum temperature for growth than adults. 
The figure below shows a comparison of optimum temperature between carp, Cyprinus carpio, and brown trout, Salmo trutta (from Elliot, 1981).
Unfortunately, information on temperature optima of ornamental fish is scarcely available. However, we can get an indication for these temperature optima is we look at the average water temperature of a fish’s habitat. The table below gives some examples of these average water temperatures in both temperate and tropical zones (Anonymous, 1999).
Examples of water temperatures in aquatic settings (°C)
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Temperate Zones |
Tropical Zones |
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Mountain Rivers |
Sea-level Rivers |
South American, Amazonian Rivers |
South-East Asia, marshy zones |
Africa, The Great Lakes |
4-14/15 |
6-18/20 |
23-28 |
26-28 |
25-27 |
The mean temperature most often recommended for freshwater aquaria is 25°C, and small variations are of little consequence. Most tropical ornamental fish species will thrive at this temperature, although some species may require somewhat lower temperatures (remember the hillstream loach discussed in the Oxygen article!), while others may require a somewhat higher temperature. The discus (Symphysodon sp.) is an example of this.
What any aquarist does need to avoid are fast and large temperature changes. These may result in thermal shock, which might kill the fish. Slow changes are generally not such a problem. Poikilotherms often have a more complex metabolism than for instance mammals. For an important chemical reaction, poikilotherms may have four to ten enzyme systems that operate at different temperatures. However, changing to a different enzyme system takes time. In addition, adjustments of lipid, haemoglobin, and hormone levels are also necessary. Therefore, complete acclimation to a given temperature needs to occur gradually and might require up to 2 weeks.
References
Elliot, J.M., 1981. Some aspects of thermal stress on freshwater teleosts. In: Stress and Fish. Ed. A.D. Pickering. Academic Press. London, pp. 209-237.
Anonymous, 1999. The Complete Aquarium Guide - Fish, Plants and Accessories for your Aquarium. Konemann, Cologne, pp. 288