Chlorine High and Algae still growing

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rizzolorf

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Aug 10, 2013
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Hi, I have been maintaining my pool for years but finally decided to join TFP because of frustration with algae. My pool water has been clear most of the summer but recently algae has started to grow rapidly even though my levels seem to be stable. I shocked it last week with 5 lb Ca hypochlorite. It seemed to help for a few days but then algae started growing again even with FC 10, TC 10, CH 250, Ph 7.2, TA 120, CYA 100. Had the auto vacuum on for the last 24 hrs to pick up algae and now pool is quite cloudy. Test kit I'm using is AquaChek test strips. What am I doing wrong?

30K gal IG vinyl, Hayward S-240 sand, 1HP Whisperflo
 
Welcome to TFP! If you'll see the chlorine/cya chart in pool school, you'll quickly discover why 10 ppm of FC ain't gettin' the job done. You've added " store shock" but haven't actually SLAMmed the pool. Oh yeah, we have a topic for that too! Test strips are notoriously inaccurate and we have test kits we recommend in pool school. A partial drain and refill is what we'd recommend.
 
Welcome to TFP. :wave:

You have a lot of reading to do. It all starts with Pool School.

A few things should jump out at you while you are reading:

  • 1. The Chlorine/CYA relationship. A CYA level of 100 will make pool management very challenging
  • 2. The use of powdered/puck forms as your source of chlorine can have some undesirable side effects
  • 3. Obtaining a reliable test kit
 
rizzolorf said:
Hi, I have been maintaining my pool for years but finally decided to join TFP because of frustration with algae. My pool water has been clear most of the summer but recently algae has started to grow rapidly even though my levels seem to be stable. I shocked it last week with 5 lb Ca hypochlorite. It seemed to help for a few days but then algae started growing again even with FC 10, TC 10, CH 250, Ph 7.2, TA 120, CYA 100. Had the auto vacuum on for the last 24 hrs to pick up algae and now pool is quite cloudy. Test kit I'm using is AquaChek test strips. What am I doing wrong?

30K gal IG vinyl, Hayward S-240 sand, 1HP Whisperflo
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You're using test strips, which aren't reliable enough

You only raised the chlorine level high enough to maybe get ahead of the algae growth once. It is a sustained thing. Much like taking antibiotics. You feel better in a day or two, but you have to keep taking them for several more days to be sure it's all gone.

You used Cal-hypo, which increased the CH level unnecessarily. Plain bleach or liquid pool chlorine is the best bet.

You've used trichlor pucks and/or dichlor "shock" in the past, which have elevated the CYA level, which renders the chlorine ineffective. You'll probably need to replace a bunch of water to get rid of that, but I'd hold off until I had trustworthy test results.

First step: a proper test kit. It's a chunk o'change, but consider it an investment. It will pay for itself in a month or so, and you'll still have reagents left for next season. Best bang for the buck: TF100 with XL option. I also recommend the speedstir. Nobody who has one regrets buying it.

Step Two: get inspired by reading some of these accounts:
first-time-shocking-with-bleach-t45074.html
pretty-black-pool-t58442.html
first-time-pool-owner-t61565.html
a-little-encouragement-for-those-with-algae-and-new-to-bbb-t57137.html
frog-filled-green-swamp-to-oasis-work-in-progress-t48213-20.html
before-and-after-t36785.html
a-final-picture-set-of-how-well-the-bbb-method-works-t33199.html

Step Three: Go back and look at some of the links in those threads in Pool School.
 
You are probably going to have to drain at least half of the pool and fill before you get anything accomplished if your cya is 100ppm. If you dont and if you eventually get it clear it will be right back to haunt you. Most people that report 100ppm cya actually ends up being way more than that. I see your using test strips and the color shade difference between 100ppm, 125ppm or 150ppm will be probably be marginal. Your best bet is to get a more accurate cya reading so you will know how much to drain, fill, and slam.
 
OK, I did get the TF100 and used the pool calculator which was very helpful. Thanks for your advice. Pool is now clear and I have accurate readings: FC 29.5, CC 0.5, CYA 70, TA 110. Next question is what is the safe FC level to swim with CYA 70 and if it's too high how can I bring it down quickly without screwing something else up? Also, why do Cl pucks raise the CYA? I don't see that in the ingredients.
 
rizzolorf said:
OK, I did get the TF100 and used the pool calculator which was very helpful. Thanks for your advice. Pool is now clear and I have accurate readings: FC 29.5, CC 0.5, CYA 70, TA 110. Next question is what is the safe FC level to swim with CYA 70 and if it's too high how can I bring it down quickly without screwing something else up? Also, why do Cl pucks raise the CYA? I don't see that in the ingredients.
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Sodium Thiosulfate will bring the FC down. Sulfates can also cause errosion/damage to concrete and plaster, but you have a vinyl pool so you shouldnt have to worry about that. Hydrogen Peroxide will also bring it down.

Forms of Chlorine
------------------------
Tri-chlor (Trichloro-s-triazinetrione) - Generally pucks/tabs but also come in a powder. It is slow dissolving and adds chlorine and CYA. They come in handy if you go on vacation or when you cant maintain your pool for a few days or if you are a little low on CYA.

Di-chlor (Sodium Dichloro-s-triazinetrione) - Comes as a powdered shock. PH neutral and adds chlorine and CYA. This actually will add more CYA than Trichlor. Useful when opening your pool and you need to shock it and your real low on CYA.

(Calcium Hypochlorite) - Generally comes in a powder for shocking but also they make pucks. The calcium hypochlorite pucks will add chlorine and calcium so it will raise the CH of the water. Usefull if your CH is low.

Bleach or Liquid Chlorine (Sodium Hypochlorite) - Adds chlorine and salt. Usefull when your water is properly balanced so you dont add unnessesary chemicals that may unbalance your pool.

(Lithium Hypochlorite) - Very expensive and really has no benefit over using bleach or liquid chlorine.

Chlorine Free Shock (Potassium Monopersulfate) - Although not chlorine, I do not know much about this product except that it is a sulfate and not a replacement for chlorine.
 

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Most of these industry sources neglect to account for the fact that the usage/consumption of chlorine is acidic so while Dichlor is fairly neutral upon addition, it is net acidic when accounting for usage/consumption. It's the same reason that hypochlorite sources of chlorine raise the pH upon addition, but after the chlorine is used/consumed, the pH drops back down so such sources are closer to pH neutral. See this post for technical details. Most pH rise in residential pools using hypochlorite comes from carbon dioxide outgassing that can be minimized by lowering the TA level.

Instead of quoting from often inaccurate industry sources, use the Pool School where, for example, the Definitions and Abbreviations refers to the acidity of Dichlor that lowers the pH and TA (though Trichlor does so more). As for side effects from the chlorine sources, the following chemical facts are independent of concentration of product or of pool size:

For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.
For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.
For every 10 ppm FC added by Trichlor, it also increases Calcium Hardness (CH) by at least 7 ppm.

Additional info of this sort may be found in Certified Pool Operator (CPO) training -- What is not taught that also describes how much the TA drops from each of the chlorine sources.
 
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