Felt is the way to go if you want crystal clear water. Like Mojo has said, they can be easily washed. I run a bleach cycle & let them dry; they come out nice & clean & I don't have the detergent expense. Dave's "hose 'em down" method would work too.
Also, not all socks are created equally. I tried a cheap one & they're very thin. The one's I have now are much thicker, but are tripple the cost. I'd like to see or buy a few of the ones on the site Grouper Dave listed to see the quality. Yep, sadly I'm a filter sock snob!
Mesh is pointless, imo.