My pool turned green AFTER adding chlorine. Why is that?

I've noticed this a lot on estimates. So much so it's probably the most puzzling thing to people. They are perplexed why their pool turns green after adding a bunch of chlorine to the water. I'll touch on a few reasons for why this happens. In fact the featured photo is a client's pool that had this same issue. He was at his whit's end because he didn't understand it.

High PH: Chlorine works better at a lower PH. The recommended PH level is between 7.2-7.8 with 7.5 being ideal. Chlorine works more effectively at 7.2 than it does at 7.8 even though a PH at either of those two are considered acceptable. So when your pool's PH is at 8.0, 8.2 and above it's not working as efficiently. This would rarely be the reason why your pool would turn green but it certainly is a contributing factor.

Filter Run-time: If you pool is not turning the water over at least once a day then it could lead to your water turning green. We really need all of the water to cycle through the filter daily to clean it in order to rid the water of contaminants. This holds true especially in the summertime. Generally speaking we need the filter to be running 8-10 hours a day during the summer time mainly because to the higher temperatures and higher bathing loads. Both are responsible for eating up your available chlorine which can turn into a green pool. In the winter time you can get a way with 4-6 hours. So make sure you have your pool system running adequately each day.

Not Enough Chlorine: I know this seems laughable but it does happen. I took over a client who's previous company stated they had a 8,000 gallon pool. After I measured it out it actually was 14,000 gallons, nearly double. So it was impossible for them to put enough chlorine in the water for their sized pool because they were dosing for something nearly twice as small. So make sure you're putting enough chlorine in your water to keep it clear!

Too High Cyanuric Acid Level: This is probably the NUMBER ONE reason people have green pools after adding chlorine. In fact, this exact situation happened with 2 newly acquired accounts that we are fixing this weekend. Cyanuric Acid (stabilizer, conditioner, CYA) is essentially sunblock for chlorine. Too little of CYA and the chlorine burns off pretty quickly which can lead to a green pool. However, too much of it and it basically turns into handcuffs for chlorine and not allow it to work properly in sanitizing your water. The recommended range is 30-50ppm. If you're approaching 100 or even more it's time to correct it. The only way to reduce CYA is through water replacement which means a partial drain and refill. There is a specific relationship between CYA and chlorine. Optimally, your chlorine should be 7.5% of your CYA reading. So, if your CYA is 110ppm that means you need to keep your chlorine at a minimum of 8ppm to keep that relationship held true. That is double of of the recommended chlorine levels of 1-4ppm. See where I'm going with this? If you kept your chlorine at 4ppm you'll get a green pool.

How does CYA get too high? Well there are two types of chlorine; stabilized and un-stabilized. Un-stabilized is liquid chlorine. Stabilized is the trichlor pucks (3" tablets) or granular dichlor. The stabilized chlorine has CYA in it. So when you're shocking your water or putting an endless supply of hockey pucks in your pool you're adding CYA to the water and may not even be aware of it. Overtime, you will run into this situation because the stabilizer is too high. So just be aware of what you're putting in your water. I'm a big advocate for liquid chlorine because you are not adding CYA when you sanitize your water. Stabilized chlorine is great for certain applications, but your standard liquid chlorine is the best for daily/weekly sanitizing.

So there you have it. That's not all of the reasons why but certainly some more of the main culprits of green water after adding chlorine.

Clear Up Your Water NOW!

6 Responses

  1. Laura
    I'm so glad I came across this. I'm a new pool owner and have never heard of CYA. I tested for it and it was well over 100. What do I need to do? Thanks. Laura
    • Laura what I would recommend is doing a partial water replacement (drain and refill). It really is the only option as there is not a chemical to reduce CYA. Companies have tried but have not had any luck. Do you have the exact CYA number? With the recommended range being 30-50ppm I would do a little math to figure out a ballpark for how much water you need to drain. For instance, say your CYA was at 150ppm, for easy math. So divide your pool into 3 equal parts and you'll see that you need to drain a minimum of 2/3 of your pool to get your CYA in range (albeit on the high side). From there, fill it with fresh water and balance the other chemicals and you should be good to go! Also watch the stabilized chlorine you put in so it doesn't get out of control again! I hope this helps!
  2. I have a 2,500 gallon Index that was just put up and filled yesterday. The water was perfectly clear until about an hour after adding chlorine. Now the water is sort of pale green. I do not know the CYA level but emptying and refilling would be useless due to the fact that I'd be filling with the same water. Also I can't refill due to being on a well and right now I cannot afford to get another water delivery.
    • Hey Kari, are you using well water by chance? It sounds like you had a reaction from adding chlorine to water with high metals in it. If you're using well water, use a metal treatment (Jack's Magic) and then something like CULator metal remover to remove the metals out of the water. That should solve your problem.
  3. Most of the reasons in this article are for a pool that is green when chlorine is expected to be adequate. Not why it "turns green right after adding chlorine. I've been a CPO for 20 years, but was just surprised that my pool turned green after adding 1lb dichlor, 1lb cal hypo. Its winter in GA water temp is in the 50's. We've had some heavy rains over the past 3 months and I was not too concerned with the pool water. It was clear, not green, and otherwise clean. I'm sure the chlorine was zero. It wasn't "sparkling", so I decided to add some Cl. I added 1 bag of dichlor (and one cal hypo) as I knew the recent rains had diluted my stabilizer (its now at 30ppm CYA and chlorine is now 5ppm). But within a couple hours after adding these yesterday it turned olive green! Profoundly different. Its my own pool that I've managed for 4 years. The symptoms suggest copper, but I don't know where Cu could have come from. I have not added any well water (probably in a year) and I've never added any algaecides. nor had any metal issues on this pool or another pool on a well 3 miles away that I've managed for 20 years. I don't have a cu test yet. Its puzzling and I'm torn between shocking it outright or checking for metals. Suggestions?
    • Thanks for the post. I'm assuming the green that it turned is a clear green? You're right, that does suggest copper. You said that you haven't added well water (in about a year) but the water you did add could have put some copper in there. Then when you spiked the chlorine up it oxidized the copper. What was your pH prior to shocking the pool? A higher PH will cause things to start falling out of solution, like copper for instance.

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