Often cat (and dog) owners report that scum or froth appears in the water reservoir in the water fountain. The problem is caused by saliva when your pet or pets are drinking. It stands to reason that some animals including dogs will produce more saliva hence the froth in the bowl. This applies to all fountains, plastic, ceramic and stainless steel.  Bio-film forms on the surface of water and looks like a scummy film on top of the water. You may have seen this on the surface of a pond. This is also evident in bowls of water not just water fountains. Ensure that all cleaning procedures suggested by the manufacturer are followed to keep the fountain in tip top shape.