Most pond owners only know of two problems related to water temperatures. In winter it gets so cold that the pond freezes over and in summer the water becomes so warm that the fish come gasping to the surface for lack of oxygen.
But there are no problems; there are preventive measures and solutions!
The first thing to ensure in winter is that the layers of water are NOT mixed, so that the 4°C warm water collects at the bottom and the fish (and other animals) can spend the winter resting there. A pump at the bottom would circulate this water and the water at the bottom could freeze. So make sure to turn off the pump in winter, or at least make sure it NEVER reaches the deepest levels. The higher density of the warm, 4°C water will make it sink to the ground.
In summer warm water can bind less oxygen than cold water. At 20 °C water can bind e.g. about 9 mg/l oxygen depending on the air pressure. At 30 °C the value drops to 7.5 mg/l. At 20 °C a fish has an oxygen requirement of 90 mg per kilo body weight/hour. When eating, the oxygen requirement increases to 500 mg/kg fish/hour!
Therefore, when outside temperatures are high it is very important to shade the pond a little (awnings, water lilies), to aerate it ( JBL Kit PondOxi ) except at the deepest point, to avoid mixing the water, and to feed the right food ( JBL PROPOND SUMMER S ) so that the animals get easily digestible food.