Temperature

Rainbowfishes are poikilothermic animals, that is, their body temperature is the same as, or 0.5 to 1°C above or below, the temperature of the water in which they live. The metabolic rate of rainbowfishes is closely correlated to the water temperature: the higher the water temperature, the greater the metabolism and their need to take in nutrients, although it probably declines in most species as they reach their upper thermal limit. The temperature of their environment is a major and even the deciding environmental factor in determining growth rate, metabolism, and nutritional efficiency. However, the amount of energy used up in finding and digesting more food at these temperatures means that growth is not commensurate with the higher rates of feeding and digesting. In tropical waters, which have prevailing high temperatures, rainbowfishes generally grow faster, mature younger, and have a shorter life span than rainbowfishes in temperate waters. Rainbowfishes do, however, display a wide range of sizes, growth rates and life spans in captivity. Water temperature also has a great influence on the initiation and course of a number of fish diseases. In fact, temperature will influence all biological and chemical processes in an aquarium.

In their natural environment, rainbowfishes are exposed to temperatures that vary considerably. This is dependent on the size and depth of the body of water, water flow, whether exposed to the sun or in shaded rain forest streams, the time of day and the season. A deep shaded backwater, for example, can be strikingly cooler than a shallow section that is exposed to the sun. The overall range has been reported from a low of 5° to a high of 38°C, and even higher in a shallow body of water exposed to full sun at midday. Temperatures in these water bodies commonly fluctuate from 8–12°C each day.

It should be noted that the survival rates for rainbowfishes in their natural habitats declines sharply when the water temperature is high and will often die at temperatures above 36°C. Such increases in temperature are common in tropical waterbodies of Australia during the late dry season. Although, the cause of death is more likely to be caused by lowered oxygen levels rather than higher temperature levels. For example, a 10°C increase in temperature (a change commonly recorded in tropical streams in northern Australia) can reduce oxygen concentration by over 2.5 mg/L, which may represent a quarter of the total oxygen present. Dissolved oxygen and temperature are critical water quality parameters and extreme levels of these parameters can cause fish kills.

The temperature of the rainbowfishes natural environment can be subdivided into extreme, tolerable and preferred ranges. In the extreme temperature range their survival time is strongly dependent on the exposure time. In the tolerable range, the fish survives without temperature-associated problems, while the preferred range is thought to be the range where physiological processes are optimised. Each species has a preferred or optimum temperature range where it grows best. At temperatures above or below optimum, their growth is reduced and mortalities may occur at extreme ranges. Rainbowfishes can tolerate the seasonal changes in temperature in their natural environment. However, these temperatures fluctuations are not recommended for aquarium keeping.

In their natural environment, rainbowfishes can search for more favourable conditions by moving into cooler or warmer water. However, in captivity this is not possible. In captivity, a temperature range of 22–24°C is recommended while an increase to 28°C can be used for breeding purposes. However, these changes should not be abrupt; temperature shock can occur if the fish are put into a new environment where the temperature is colder or warmer than the original water. Under these conditions fish may die, showing symptoms of paralysis of the respiratory and cardiac muscles. With young fry, problems may arise even where the difference in temperature is as low as 1–3°C.