Algae spores constantly make their way into almost all pools. The wind may blow in algae or small amounts may stowaway on swim suits and pool toys. Regardless of how they entered your pool, a few spores can multiply into a bloom of single-celled plants seemingly overnight. These blooms are especially frustrating to deal with because even once you have killed the majority of the algae, the remains of the tiny plants can continue drifting around the pool.
Follow the instructions of your PH testing kit to ensure that your pool's PH is balanced. Each kit has its own procedure that usually involves little more than testing a small quantity of water, some simple math, and the addition of either an acid or base to the pool. This is important because it will inhibit further algae growth. Set your filter to run constantly.
Add enough chlorine to the pool to raise its concentration to ten parts per million. Thoroughly brush the walls and floor of the pool toward the drain. Make sure that you brush every surface of any lights or ladders in the pool. Leave the filter running constantly until the pool's chlorine level has dropped to five parts per million.
Add algaecide to the pool. The amount will depend on the concentration of the algaecide, but the bottle should come with instructions for recommended dosages. Re-brush the sides and bottom of the pool, directing as much as you can of the algae toward the drain. Continue running the filter until the algae settles to the bottom of the pool.
Use your pool's vacuum attachment to remove the dead algae remaining on the pool floor. If you have an easy means of refilling the pool as you vacuum, you can pump to waste to ensure that the algae the vacuum is picking up is completely removed from the system. If you have to use the filter while vacuuming, vacuum until suction diminishes, then switch the filter to backwash until the water being pumped from it runs clear.