During hot spells of weather large quantities of water can be lost from a pond due to evaporation. This can be up to 10cm per week in exceptional conditions. It is best to replace the evaporated water on a regular basis using water from a tap or other sources (rain water, well etc). In this way you avoid any large changes in water conditions and also don’t get the water lilies submerging. If only replacing a small amount of water it shouldn’t be necessary to use a tap water conditioner – but if adding any more than 10{401015f8f8e2343b4069050f37ca81dccf45ad1a501f298b5d2086063e4e83fa} of the pond volume always use a good quality de-chlorinator. When adding the water don’t allow any splashes to get into open water lily flowers. They don’t like it and will not last very long if they get wet.
Water movement from a fountain or waterfall will increase evaporation. If it is windy the water loss can be even greater as some of the water will be blown out of the pond. Although a problem, only turn the fountain off in severe conditions and always leave waterfalls on. In hot weather the fish need the added oxygen that the waterfall or fountain add to the pond and turning them off can cause deoxygentaion, particularly during the night.
In a warm climate, adding water lilies to cover part of the surface will minimize evaporation.