Generally, we agree on 24°C.
For the Amazonian biotopes, we often go even further.
An inappropriate temperature is obviously a source of stress for the body.
The temperature of their body varies with that of the water.
Each species has its optimum.
A goldfish outdoors goes from 5°C in winter to 25°C in summer.
But a trout will die if the water is warm, less from the heat than because warm water is too low in oxygen.
Conversely, a Discus cannot tolerate cool water and will become ill, then die, as its metabolism fails at low temperatures.
Within a certain limit, temperature can appear as a relatively secondary factor.
For example, a tropical fish, placed outdoors in summer, will choose its temperature by swimming to the surface or diving deeper.
Indeed, there can be a temperature difference of 6°C between the surface and the bottom of a poubellarium in summer.
However, it will not hesitate, to avoid your gaze or to chase a daphnia , to dive in a matter of seconds, thus easily enduring a significant temperature difference.
I personally picked up guppies, platys, and swordtails at the end of October in water at 11°C.
They were in perfect health, with unmatched colors and an incredible vitality in the net.
They would never have survived that in an aquarium.