I try and remember to use the Brita filter, but as two of the water bowls are upstairs, they do get tap water most of the time.
Kimberley, I agree with Ela - you should be worried if your cats aren't drinking enough water on dry food, they could be at risk of health issues. Our water here isn't too bad at the moment, but we have had probs with it in the past, and I wouldn't drink it then, so wouldn't expect my cats to!!