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How to Prevent Your Pool from Turning Green: By Tom Queen

Updated: Jun 20


Weather is heating up and with that pool season is quickly approaching. The last thing any pool owner wants is to walk outside one morning and find their pool has turned into a swamp. Here in Florida, a green pool can be a fertile home for frogs, snakes, and even the occasional alligator. Despite being an eyesore, the water in a green pool can usually be turned back to crystal clear blue again with some work. But the best way to deal with a green pool is to prevent it from getting that way in the first place.


The cause of a green pool is very simple, Algae! When there is a lack of sufficient parts per million of sanitizer in the water, the pool is an open invitation for a green algae bloom. Low levels of sanitizer in the water combined with high levels of phosphates, add to that warm water and or a high PH (7.8 +) and you have a recipe for a swamp. If you have a salt system, you must keep your salt level in proper range (usually around 3400 -3600) but check with your owner's manual. Pool service professionals like myself rarely pay any attention to phosphate levels because with adequate sanitizer in the water no algae can bloom. However, for a pool owner, reducing phosphates in the water can often help prevent an algae bloom should the sanitizer level get low for a time. Phosphates are basically the food that algae feed on in the pool. Be warned however, that if you choose to use a phosphate killer in your pool, the water will likely turn cloudy and require either pool clarifier or a flocculant to corral the dead phosphates suspended in the water so they can be vacuumed or filtered out.


If your pool does turn green, it will be necessary to either shock it or drain and replace water. Shocking a pool is also called breakpoint chlorination. What it means is simply raising the amount of free chlorine in the pool to a high enough level to destroy the algae, bacteria, and chloramines. But before you do that, you should understand the dynamics of using chlorine as a sanitizer. When chlorine is added to water it forms hypochlorous acid and a hypochlorite ion. The hypochlorous acid is the powerful form that kills the algae and other contaminants in the water. The hypochlorite ion is the weaker form of chlorine. The ratio of hypochlorous acid to hypochlorite ions is determined by the PH of the water. The higher the PH, the less percentage of hypochlorous acid and the higher percentage of hypochlorite ions. This means that as the PH goes up the chlorine gets weaker and more ineffective; Conversely, as the PH drops the more powerful the chlorine becomes. Pools are ideally kept within a PH range from 7.2 to 7.8. At a PH level of 7.2 chlorine works at about 80%, whereas at a PH level of 7.8 it only works at about 20%. So, before you go about shocking the pool you will want to know what the PH level is and possibly need to lower it. But even before you measure the PH of the water there is one other test that I always do first. This is the most important test because it determines if you can simply shock the pool back to blue or if you will need to at least partially drain some water.


Cyanuric Acid otherwise known as (CYA). CYA builds up in water over time and eventually neutralizes any free chlorine added, this is called chlorine lock. The peculiar thing is that different pools reach chlorine lock at different levels of CYA. CYA is a chlorine stabilizer and necessary to attach to and hold chlorine molecules in the water initially. However, over time the accumulation of too much CYA will completely block any free chlorine's ability to work. Ideally, CYA should stay between 30 - 90 ppms. If CYA is over 90 at least a 1/3 drain of water is recommended. This will reduce the amount of CYA in the water and allow you to shock the pool. If the pool is in chlorine lock, a full draining and water replacement may be the best option. If CYA is relatively low - 90 or under, you can safely assume that you will be able to shock the pool back to blue.


To shock the pool effectively you must have at least a good estimation of how much water is in the pool. If you don't know exactly how many gallons your pool holds, there are pool calculators online that can help you estimate the gallons of water by measuring the pools approximate length, width, and average depth. Once you know approximately how many gallons of water your pool holds, you will want to test and adjust the PH to 7.2 by adding muriatic acid. There are many calculators and charts online to guide you to how much muriatic acid you will need to adjust your PH accordingly. Once the PH is at the correct level, you will then want to open and clean your filter. If you have a standard cartridge filter, you may want to replace the cartridge rather than try to clean it. In any event a clean filter will help once you add your chlorine to shock the pool. At this point you will want to add your pool shock using a table or chart similar to the one below. You will then want to run the pump for 24 hours to get good circulation of the super chlorinated water through the whole system. Once again there are many charts and calculators online to help you with the amount of chlorine shock you will need to use.


This particular chart I found on a website and has no affiliation to me in any way. I provide this only as an example of the many resources available online that can be used. Unlike what this particular chart says, if I were shocking a green pool with algae, I would go above 30ppm. Normally, a green pool would be shocked to at least 50ppm or higher. This leaves little chance that any green algae would survive. Once the shock has been added.


Once the algae have died and the pool has turned back to blue, the water may be cloudy from the dead algae particles suspended in the water. A thorough brushing of the walls should also be undertaken and may add to the cloudiness. Often a floccing agent can be used to corral all of these particles and drop them to the bottom of the pool where they can be vacuumed out. At this point you should have cleaned your filter at least once. After vacuuming all the dead algae particles out of the pool, you will want to reclean the filter once again.


If you are doing a partial drain you would start draining the pool before you do anything else. After the pool has drained to the proper level, you will need less shock to reach the 50ppm or higher super chlorination level. Once the remaining water in the pool has cleared back to blue you would then fill the pool with fresh water and add more chlorine to maintain shock levels. Then clean the filter and run the pump for 24 hours. If the water is cloudy follow the floccing procedure above and vacuum then re clean the filter. If you do a full drain then do a chlorine wash on the walls, stairs, and floor of the pool to kill all of the algae. Clean the filter and replace with fresh water. Then you will need to add either stabilized chlorine or conditioner and non stabilized chlorine until you reach 3.0.


As you can see from the above article, the best remedy for a green pool is prevention. The most effective prevention is:

  1. Keeping good circulation of water in the pool, your pump should run for at least 4 hours a day during the winter and 6-8 hours a day during the summer depending on the climate you live in.

  2. Maintain proper levels of sanitizer in your pool at all times.

  3. If you have a salt cell, keep salt at the proper level and adjust to increase or decrease chlorine output as required. Also clean salt cell regularly.

  4. Clean your filter regularly. Maintain good filtration.

  5. (Optional) Monitor and control the phosphate levels in your pool.


If you follow these simple guidelines, your pool should stay nice and clean and algae free despite the constant battle against the elements, leaves, organic matter and debris that get into the pool.






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