the wife changes the water in those things 3/4 times a week. Doesn't matter, the only loss is from evaporation. Waste of energy/money. Yes, they prefer running water due to the added O2 that happens during the process of water mixing with air. https://www.caryinstitute.org/sites/default/files/public/downloads/curriculum-project/1C1_dissolved_oxygen_reading.pdf It's called dissolved oxygen, and all cats in the wild will seek out moving water and almost never will drink from a standing source. I'd venture it has to do with survival and bacteria growth in standing water.