Algae growth in a swimming pool may not be caused from too low a level of chlorine but too high a level of phosphate. Phosphate is the main source of food for algae. Phosphate comes from decaying plant life, such as leaves that fall into the pool. It can also come from detergents or fertilizer that may find their way into the pool. Decreasing the level of phosphate in a pool is accomplished with an inexpensive chemical and back-washing.
Test the water in your pool with a phosphate test kit. Most pool chemistry test kits do not test for phosphates, so a special kit will have to be purchased for this purpose. Follow the instructions on the kit you purchase for testing the phosphate level. An acceptable level of phosphate to keep algae controlled is below 100 parts per billion.
Add a phosphate elimination chemical to the pool water per the instructions on the bottle. The phosphate remover binds with the phosphate to make it a solid so that it is removed when you back-wash the pool.
Back-wash the pool according to the manual that came with your pool. Pools can vary with how they are back-washed, so knowing the procedure for your pool is essential. You may need to back-wash the pool several times to remove the solid waste of the phosphate.
Test the phosphate level of the water on a weekly basis with the test kit, and use the phosphate remover and back-washing when necessary.