...I'd write a long post, but I promised the cats I would swap out their water fountain for a freshly-cleaned one and I'm at least a day late already!
The water around here is very hard, so the little running-water dishes have to be cleaned several times a year.
ETA: Out of four "Drinkwell" cat-waterers (two old style saved for parts, two recent ones) we appear to have exactly one lid for the filter compartment. How does this happen? I don't know.
5 months ago