At some point in time, if you own a pool, you will have to deal with algae. Algae are deposited into your pool water by the wind, rain, pool toys and many other things that come in contact with it. Keeping it from becoming a problem is not as simple as one may think. You may not even notice that your filtration system is not running at top performance, or you may go on vacation and have very hot and humid weather while you're gone. These and many other factors can lead to algae growth. However, you can get rid of it, most of the time, without emptying your pool.
Balance the water. Use your water test kit and make sure the pH and chlorine is where it should be. PH should be between 7.2 and 7.6; if it's not, add some pH plus or minus to the water, circulate and check again in an hour.
Clean the filter. Backwash and make sure your filter is free of clogs. This will help filter out the algae better. Allow it to run 24 hours a day from this point until the algae is gone. Keep any automatic cleaners working in the pool.
Shock the pool. Fill a bucket halfway with water and add the shock to it. Never the other way around or you could cause the shock to puff up into your face. Put in one pound of shock for every 7,500 gallons of water in the pool. Mix and pour into the pool. You may want to wear gloves and a face mask as the chlorine shock is very strong.
Brush the algae off the walls and the bottom of the pool. Keep an eye on the filter pressure gauge and backwash as necessary. The pool may become so cloudy that you can no longer see the walls and bottom. If this occurs, allow the filter to run for awhile and the algae to settle, then repeat.
Add a polymer algaecide. This will both kill the algae and capture it, making it easier for your filter to get it out of the pool. Vacuum if you don't have an automatic vacuum in the pool. Check to see if the filter needs to be backwashed again.
Allow the chlorine level to come back down to normal, 1 to 3 ppm, and clean out your filter. Add potassium tetraborate to the pool for the prevention of algae returning. Follow manufacturer's suggestions for amount according to your pool water capacity.
Balance the pH again. Keeping the chemicals in balance and using a preventative algaecide will help keep the pool clean. Heat, sun or lack of sun and rain can cause the chemicals to be used up faster than expected. Test often under these conditions.