Green to Clean: How to Clear up a Green and Cloudy Swimming Pool
Help! The Pool Is Green!
Treating a pool that has turned green? This article will get you step-by-step through how to get that nasty pool in shape. I will also cover some basic chemistry and filtering tips to prevent this from happening again.
All information is based on an in-ground home pool of average size, from 12 to 15 thousand gallons.
What Makes the Pool Green?
Algae make it green, and a chemical imbalance lets the algae grow. If there is enough free chlorine in the pool, algae will have a hard time forming. If for some reason the chlorine can’t build up to a high enough level, algae will grow. That's why you shock your pool with chlorine as part of the six-step process below. If the green comes back, you can adjust the levels of other chemicals besides chlorine, as there is more to pool water chemistry than chlorine alone.
Six Steps to Cleaning a Green Pool
- Determine whether your water chemistry can be fixed, or whether it's too far gone.
- Test the water to determine the pH balance.
- Shock the pool.
- Pump and filter the pool to regain balance.
- Brush and filter the pool again.
- Maintain balanced chemistry.
Each of these steps is described in detail below. All information is based on an in-ground home pool of average size, from 12 to 15 thousand gallons.
Step 1: How Green IS Your Pool?
Because if it's too green, these six steps won’t be enough; you may need to have the pool drained and acid-washed, instead of shocked. I have seen many pools that were not just green, but black. In severe cases like this, it is more cost-effective and less time-consuming to simply drain the pool and have it acid-washed, even though it costs money to refill the pool.
This is my general rule for determining whether the pool can be treated chemically or needs to be drained: if you can see at least six to eight inches below the surface of the water, most likely the pool can be treated chemically. An example is in the photo below; you can see the top of the first stair down into the pool.
Once we establish that the pool doesn't need to be drained and can be treated chemically, we can go from there.
Step 2: Testing the Water
If the pool is green, there’s obviously little or no chlorine in the pool, and if you are going to shock it you will be adding a lot, so testing for chlorine is not very important. But you should test for pH, because if the pH is very high, the shock will turn the pool cloudy. A shocked pool will be cloudy anyway until all the dead algae and other solids are filtered out, but high pH will cause a VERY cloudy pool.
To find out the pH, I prefer to use a high-end test kit, but the cheap test strips will at least give you an idea if your pH is high or low. You want it to be low: 7.2 or lower.
If the pH is high, add one gallon of muriatic acid, which should be enough. Don't worry that you may have added a little too much, because the pool can be a little acid (low pH) for swimming purposes but still at a good pH for shocking. Test the pH again after shocking and four hours of circulation.
Step 3: Shocking the Pool
Once your pH is 7.2 or below, you are going to start by shocking the pool with granular chlorine (calcium hypochlorite). I suggest purchasing a 25-pound container of granular chlorine, rather than the individual one-pound bags they sell at pool stores or large chain stores. You'll save a great deal of money, and you will need chlorine in the future for small doses from time to time. Use five pounds of granular shock, or 10 gallons (four 2.5-gallon jugs) of liquid chlorine. With the filter pump on, broadcast the chlorine evenly over the water, covering the entire pool surface, until all five pounds of shock, or all ten gallons of liquid chlorine, have been used. Be sure to use a good algaecide as well, which you can add after a few hours of circulation. You can also add a floccing agent at this point to cause the dead algae particles to clump together.
I wrote a follow-up article about shocking the pool that answers some of these questions in more detail:
- How much shock should I use?
- How long should I run the pump after shocking?
Step 4: Pumping and Filtering
What type of filter do you have? Follow these steps for your type of filter.
- Diatomaceous Earth (DE) Filter: Before doing anything else, backwash the DE filter. Add fresh DE powder. Then shock the pool as explained above, and run the pump for 24 hours. Make sure the pool drain is not obstructed by leaves or other material while this pumping is going on. If the pool is so green you can't see the drain, just run a brush over the approximate location of the main drain at the deep end of the pool.
- Sand Filter: Same as for a DE filter, except the backwash time should be a minimum of five minutes.
- Cartridge Filter: Make sure it's in good condition and rinsed thoroughly. See note in Step 4 below about cartridges.
Step 5: Brushing and Filtration
After 24 hours of chemicals and circulation, you will see an amazing transformation. Your pool should not be green anymore. But it will still be cloudy, and it will need a lot of brushing and filtration for the next few days. There will probably still be a few stubborn isolated green areas that need some brushing.
It will take a while for the cloudiness to go away.
- For a sand filter, it will take a week or more.
- Cartridge filters will need to be cleaned more often than other filters during this process: twice a day for at least two days, or until the pool is clear.
- If you have a DE filter, and the cloudiness does not clear up, your filter may be clogged and need repair or maintenance.
Step 6: Maintaining Your Pool in Balance
Tips for maintaining your pool:
- Make sure you have a reliable chlorinating system, whether it is an in-line, floater, or salt system. Chlorine needs to be in the pool always. Throwing a jug of liquid in it once a week isn't a good way of maintaining the pool. A simple tablet chlorine floater is very effective.
- Use a water clarification solution.
- Clean your filter. DE filters are by far the best filter to have. Although a bit more costly to purchase at first, they will save you both time and lots of money in the long run.
How often to clean your filter:
- DE filters: Backwash once a month.
- Sand filters: Backwash once every two weeks. Be sure to backwash your sand filter for a minimum of four minutes; otherwise, you will see filthy water shooting back into the pool.
- Cartridge filters: Clean it every three to four weeks, unless you see algae in the pool, in which case you should clean more often. Here's more about how to clean a cartridge filter. Soak in trisodium phosphate every three months.
Do you maintain the pool yourself?
Questions & Answers
Why did my pool turn green after shocking?
This is a chemistry issue. If the pool turned green immediately after shocking the pool, then there is an imbalance with other chemicals or something else has been added to the water that should not have. If the pool has been brushed and shocked, it will take a few days to filter out the "green" and for water clarity to improve. If the pool has been shocked and is no longer green, but turns green again after a day or two, check the water chemistry and pay particular attention to phosphates in the water. Clean the filter often during this process. After shocking the pool, filtration becomes a key factor in clearing up a pool.
The pool store showed copper level of .3 which they said is what is making my pool green. PH was high and chlorine was low. I poured a bottle of metal free in and after 24 hrs, no change. Any suggestions?
I would question the integrity of the particular pool store you visited since copper will NOT turn a pool green. It sounds as though you were the victim of an "upsell". Although it is good to keep your pool free from metals, this is not why the pool is green. Low chlorine and high pH makes more sense. Balance the chemistry (pH, Chlorine and alkalinity) I would recommend the pool also be tested for phosphates. Make sure the pool filter is clean and working properly with good flow. (Cartridge filter element need replacing?, Sand filter working right? DE filter holding DE powder?) If you have an in ground pool with a concrete type surface with a serious copper problem, then blue or teal color stains will be visible. A vinyl liner will be difficult to detect any "metal" type effects.