Use a chemical test kit to test the chlorine and pH levels and determine the extent of the problem. When chlorine levels drop below 1 ppm, it can cause algae to grow in the pool, turning the pool water green. When this happens it is necessary to "shock" the water with chemicals to kill the algae and return the pool to normal chlorine levels.  Proper pool maintenance, including having working filters and making sure your pool's chlorine and pH levels remain steady, can prevent algae from growing in the first place. Algae is constantly growing, so letting your pool sit without maintenance for even a few extra days might create a green pool water situation. . Before treating the pool, balance the pH by adding either an acid or a base to bring the level to just around 7.8. This is at the high end of the range you would normally want in your pool, but that's necessary when you're treating it for algae. Here's how to balance the pH:  Turn on your pump so the chemicals will circulate throughout the pool. Correct the pH level by either increasing the pH with sodium carbonate or decreasing it with sodium bisulfate. Clean out any leaves, sticks, and other debris that may be clogging the filter.  Backwash the filter if necessary and make sure it is working well before you add chemicals to the pool to kill the algae. Set the filter to run 24 hours per day so it filters out all the algae during the cleaning process. Use your pool brush to scrub the pool well before adding any chemicals to the water. Algae clings to pool surfaces, but scrubbing will remove it. Scrubbing also helps to break the algae up, allowing chemicals to work faster.  Scrub especially well in areas where you can see algae buildup. Try to break it all up so that the pool gets thoroughly clean. If you have a vinyl pool, use a nylon scrub brush. Wire brushes might damage vinyl pools, but may be used on plaster pools safely.

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Write a summary of the previous text in Chinese:
检测池水。 使池水达到化学平衡。 确保过滤器正常运行。 刷洗泳池四壁和池底。