In most cases, a swimming pool algae contamination is dealt with by shocking the contaminated pool. Shocking a pool to kill off an algae contamination, though, won't clear away the dead algae left. Dead algae in a pool are so microscopically small that most will float in the water, clouding it up as fine particulate matter. Algae in general are also so tiny that not all may be trapped by a swimming pool's filter once they've been destroyed.
In a swimming pool, dead algae need to be heavy enough to settle to the bottom of the pool to allow for vacuuming. Making dead swimming pool algae heavy enough to settle to a pool's bottom is accomplished through pool clarifiers. A good swimming pool clarifier will force dead algae to clump together, giving them enough weight to allow for settling. You'll rarely find that dead algae in a pool will settle to a pool's bottom without first being treated with a clarifier.
Once you add clarifier to a swimming pool, run the pool's filter two hours before shutting it off. Running the filter after adding clarifier to a pool will allow clarifier to disperse fully throughout the pool. Leave the pool filter off for 12 to 24 hours to give a water clarifier time to clump up dead algae or organic matter. Once dead pool algae is clumped up and settled to the bottom of the pool, vacuuming using the pool's filter will be easier.
Different types of pool clarifiers can help clear swimming pool water clouded up by dead algae. All pool clarifiers, though, work on the same principle of making fine particulate matter like dead algae heavy enough and large enough to settle out. Also, pool clarifiers come in different strengths, formulations and concentrations. As little as an ounce of clarifier in 5,000 gallons of water will effectively treat a pool that's cloudy from dead algae.
Once dead swimming pool algae has settled and after it has been vacuumed up, check your pool's filter if it's a sand model. Sand pool filters can become channeled and will just recirculate water, not filter it. If the pressure in your pool's sand filter refuses to rise, there's a good chance it's channeled. Channeled pool filters can't effectively filter out any remaining dead algae. Resolve channeled sand pool filters by backwashing or replacing the old sand.