The wels is mainly inhabiting regions which are only in summer really warm, and even then temperatures over 20°C are not common. In winter the waters even often freeze at the surface. But still the wels is very well doing in warm water, in fact it really flourish when it is keept in water over 20°C. The temperature for optimum growth is about 26° C, so it would be no problem to keep a wels in a tank with tropical conditions. I have read about people which keept them in warm water, and the growth was extreme, the only problem was that they did outgrow their tanks in a very short time.