Purchase a water-testing kit from your local pool-supply store. Follow the directions that come with the kit. The kits are color-coded. The color of the water or the testing strip will determine what steps you need to take next.
If your pH levels are high, add acid. If your chlorine levels are low, add chlorine. Buy acid or chlorine from your local pool company. Chlorine comes in granular or liquid form. The liquid form works faster, whereas the granulated form appears cloudy prior to clearing. Liquid chlorine comes in a 2-gallon jug.
Call your local pool-service company and schedule a time for pool service. Pool professionals can maintain proper chemical balance and treat your pool for various algae. Some companies provide and add chemicals only for swimming pools.
Take a sample of your pool water to a local pool-supply store. Ask it to run an analysis of your pool water. The service is usually free. The results of your test will determine which chemicals will need to be added. Purchase the necessary products and follow all instructions.