Scrub with a stiff brush, then shock the pool.

Scrub with a stiff-bristle pool brush first. Algae clings to rough surfaces like steps and corners, and chemicals alone won’t dislodge the biofilm. Get in there with a brush that has angled bristles for tight spots.

After scrubbing, hit the pool with a chlorine shock (or a non-chlorine shock if you want to swim sooner). That kills any remaining spores. For stubborn white water mold or black algae, you may need a pumice stone on plaster surfaces, but test an inconspicuous spot first—it can scratch.

Don’t bother with expensive “algae spot removers.” A brush and chlorine do the same thing for less money. The secret is elbow grease.

Your pool pump won’t do the scrubbing for you.