As I understand it, clarifier is used to get particles that are suspended in your water to clump together and sink to the bottom so you can vacuum them up. I used it once when I had killed a bad algae outbreak and the dead algae was making the water cloudy. However, most recently when I killed algae (after I started using this website) I patiently waited for the dead algae to be filtered out by my sand filter. It took a week or so, but now my water is beautiful, and I didn't have to vacuum up the muck from the clarifier. I backwashed the filter, keep my Cl levels where they should be, and the pool is crystal clear!