If you have experienced heavy rainfall recently or your pool has been used extensively, your pool's pH level is most likely unbalanced, which will lead to a buildup of algae. Pool water must have a pH level between 7.5 and 7.8 so that chlorine can prevent algae from growing. If your pH level is too high, use muriatic acid to bring it down. Muriatic acid, like most pool chemicals, is extremely dangerous so exercise caution when using it.

...
Keep your pool's pH level balanced.

Step 1

Test your pool water's pH level. Follow the instructions included with a pool testing kit to get an accurate reading. The pH level should be between 7.4 and 7.8.

Step 2

Wear a pair of large rubber gloves. Muriatic acid is extremely caustic and will severely burn your skin if you come in contact with it. Turn your pool's filtration system on.

Step 3

Measure out the appropriate amount of muriatic acid. Place a small bucket on a scale and fill it with muriatic acid. Use one pound of acid for every 10,000 gallons of water if the pH level is between 7.8 and 8.0. Use one-and-a-half pounds if the pH level is between 8.0 and 8.4. Use two pounds of acid if it is over 8.4.

Step 4

Carry the small bucket of acid to the pool. Start at the deepest end of the pool and pour a small amount of the acid into the water. Walk around the pool and add some of the acid every two to three feet until all of the acid is in the pool. Wait four to six hours before getting into the pool.