An outbreak of algae can make your pool an uninviting mess that you spend more time working on than enjoying.
Unfortunately, chemicals such as shock, algaecide and clarifier that are commonly sold in pool stores will rarely get you good results. However, with a high quality water testing kit, knowledge of correct chlorine levels, and inexpensive household bleach, you can clear up your water in a matter of a few days.
(Note: this procedure is for chlorine sanitized pools only. Do not use this if you use other products such as Baquacil or Bromine).
Take a sample of your water to a pool supply store for testing. Make sure that they provide a reading for stabilizer (also known as cyanaric acid or CYA). If this reading is too low (below 30), then you might purchase some stabilizer so that you can add it to your water. Other than that, do not let the salesperson talk you into purchasing shock, algaecide, clarifier or any other chemicals.
While you are at the pool store, purchase a quality chlorine test kit. The common drop test (where you add 5 drops of a yellow liquid to the water and match the tint) is not very accurate and can only measure up to 5ppm of chlorine. To ensure that your chlorine levels are high enough to quickly kill the algae in your pool water, you will need a test that's capable of accurately measuring high levels of chlorine. The most common type is known as the FAS/DPD drop test. You can purchase the components for this test individually or as part of a complete pool testing kit such as the Taylor K-2006, the Leslie's Chlorine FAS-DPD Service Test Kit, or the TF Test Kit's TF100.
The final item you will need is conventional, unscented household chlorine bleach. Name brands are not necessary. You can purchase the least expensive brand as long as the label says "6% sodium hypochloride." the amount you need is dependent on the size of your pool and the severity of your algae outbreak. Generally speaking, buy at least 2 - 4 jugs. For larger pools or serious outbreaks, you may need more.
Set your pump to run continually. This will ensure that all of the algae is exposed to chlorine and killed completely.
Use the amount of stabilizer measured in step 1, and the chart on the left to determine the target level of chlorine required to shock your water. If your stabilizer is above 60, you may want to consider reducing it (by draining part of your water, then refilling with fresh water) so that you do not have to use excessive amounts of bleach.
Follow the instructions in your chlorine test kit to measure the amount of free chlorine (FC) in your water.
Calculate the amount of bleach to add. A very useful tool for this is located at http://www.poolcalculator.com. Enter your pool's capacity in gallons in the first row. In the next row (labeled "FC"), enter the results of your chlorine measurement under the "Now" column and your target chlorine level from step 5. Click the "calculate" button and note the amount of 6% bleach called for.
Measure out the amount of bleach called for, and add it directly to your pool water. You may pour it in front of your skimmer or pump discharge so that it spreads throughout your water quickly.
Wait at least two hours, then repeat steps 6-8 for 2 to 3 days, or until your water has cleared up.