Cat water fountain problems

We have a cat water fountain that the cats enjoy, since they like running water. Unfortunately, it seems to get cloudy, bubbly, and/or slimy very quickly. The filters we have say they should be good for about a month, but a week after changing the filter, washing the fountain and its components, and putting it out, the cats started yakking again and the water looked a bit too bubbly.

So, what should we do? Is this a sign of a bigger mold problem? Is it an indicator to throw out the fountain and get a new one? Is it a sign that we haven't kept things clean enough around the fountain? Or something else?

I'm at the point where I'm going to buy bottled water and disposable paper bowls and change them every day to avoid this happening again, so I'd appreciate any help or advice anyone can give to help with this problem.

by WorldRov3r