When your pool water is green or contains visible algae clumps, your pool does not have enough chlorine. "Shocking" the pool with a large dose of chlorine is the most effective way to kill the existing algae and bring your pool back to sanitary conditions. This usually works within 1–3 days, but can take up to a week if pool conditions are poor. The other methods listed below are faster, but may not fix underlying sanitation concerns. They are also more expensive and can have unwanted side effects. Brush vigorously to remove as much of the algae as possible. This will reduce the amount of time it takes to kill and clear the algae bloom. Pay special attention to the steps, behind ladders, and other nooks and crannies where algae tends to gather.  Make sure the brush is compatible with your pool. Steel brushes work well on concrete, while nylon brushes are preferable for vinyl pools. You'll be handling dangerous chemicals during this method. Always read the safety information on the labels first. At minimum, follow these safety standards for all pool chemicals:  Wear gloves, eye protection, and clothing that covers your skin. After use, wash hands and inspect clothes for chemicals. Avoid inhaling the chemicals. Use caution when handling in windy weather. Always add chemicals to water, never water to chemicals. Do not place wet scoops back in container. Store chemicals in sealed, fireproof containers, away from children, on separate shelves on the same level (not one above the other). Many pool chemicals explode when they touch another pool chemical. Use a swimming pool pH test kit to measure the pH of your water. If the pH is above 7.6 — which is common during an algae bloom — add a pH reducer (such as sodium bisulfate) to your pool according to label instructions. Aim for pH levels between 7.2 and 7.6 to make your chlorine more effective and reduce the growth of the infestation. Wait at least a couple hours, then test the pool again.  Test kits that use tablets or droppers are much more accurate than paper test strips.  If pH levels are back to normal but total alkalinity is above 120 ppm, check the pH reducer label for instructions to bring total alkalinity down between 80 and 120 ppm. The chlorine you use for regular pool treatment may not be the best choice for a shock treatment. Ideally, you should use a liquid chlorine product intended for swimming pools. The product should contain sodium hypochlorite, calcium hypochlorite, or lithium hypochlorite.  Avoid calcium hypochlorite if you have hard water. All hypochlorite products are flammable and explosive. Lithium is relatively safer, but much more expensive. Avoid granular or tablet chlorine products (such as dichlor or trichlor), which contain stabilizers that should not be added to the pool in large quantities. Check your chlorine product's label for "shock" instructions. To fight algae, use twice the recommended amount for a regular shock. Use triple the amount if the water is very murky, or even quadruple if you can't even see the top rung of the ladder. With the pool filter running, add the shock directly to the perimeter of the pool. (If you have a vinyl pool liner, pour the shock into a bucket of pool water first to avoid bleaching.)   Warning — liquid chlorine will explode and produce corrosive gas if it touches chlorine tablets or granules. Never pour the liquid chlorine into your pool skimmer or anything that contains these products. Because UV rays in sunlight break down chlorine, shocking is most effective when added in the evening and left overnight. After the pool filter has been running for 12–24 hours, examine the pool. Dead algae turns white or gray, and either suspends in the pool water or settles to the floor. Whether or not the algae is dead, test the pool again for the new chlorine and pH levels.  If your chlorine levels are higher (2–5 ppm) but the algae is still there, keep maintaining these levels as usual for the next couple days. If chlorine levels have risen but are still below 2ppm, shock a second time the next evening. If there was no significant change in your chlorine levels, your pool likely has too much cyanuric acid (more than 50 ppm). This comes from using granulated or tablet chlorine, and can "lock" your chlorine into unusable forms. The only way to fight this is to shock repeatedly (sometimes many times), or to partially drain your pool. Large amounts of leaf litter or other objects in the pool can also eat up your chlorine. If the pool has been unused a long time, this could take a full week and several shock treatments. Brush vigorously to fight new algal growth on the walls. Over the next couple days, the chlorine should kill the algae. Test daily to confirm that chlorine and pH levels are acceptable. A well maintained pool has roughly the following values: Free Chlorine: 2-4 ppm, pH: 7.2 – 7.6, Alkalinity: 80 – 120 ppm and Calcium Hardness: 200 – 400 ppm. Slight differences in standards are common, so a small deviation should not be an issue. Once there is no green color left in your pool, vacuum up all the dead algae until the water is clear. You may skip this step and let the filter handle it, but only if you have a powerful filter and are willing to wait several days.  If you're having trouble getting all the algae, add coagulant or flocculant so it clumps together. These are available at pool stores, but may not be worth the purchase for a home pool. If you have a D.E. filter, set it to backwash. If you have a cartridge filter, remove it and clean the cartridge with a hose at high pressure, followed by dilute muriatic acid or liquid chlorine if necessary. If you do not clean the filter thoroughly, dead algae may block the filter.

Summary:
Use chlorine as your go-to algae killer. Brush the walls and floor of the swimming pool. Review pool chemical safety. Adjust the pool pH. Choose a chlorine shock product. Add an extra large dose of shock. Test the pool again the next day. Brush and test daily. Vacuum the dead algae. Clean the filter.