We have the Drinkwell brand with the 6 cup reservoir and I clean it out, refill it once a week or so (more often when Cassy goes swimming).

I have learned that it promotes evaporation even when the cats aren't present. (We all went on a vacation and I forgot to unplug it. Got home just in time to hear the last feeble noises as the motor burned itself out. Big time EEEK!!! No body was hurt, no fire thank heavens... realatively cheap to replace motor ($15?))

At any rate, there is a lot less static electricity around the house with the fountain... for what it's worth.