We have two main water bowls, one upstairs and one downstairs. Both are stainless steel bowls. The bowl upstairs gets filled every day. They seem to drink it dry right before bedtime. The bowl downstairs is a little bigger, gets filled maybe every two days. They also have little bowls (That attach to the side walls) in their crates. Those get refilled less often, but I watch them for any kind of 'floaters'... ie hairs, backwash, build-up, etc. Never seen algae in our bowls. But I'd never let them drink it if there was.