Did algae eat my CYA


Well-known member
Jun 17, 2019
Central NH
I'm not sure if this should be here or in the algae forum.

Following the directions of the pool store/installer I had been using a dichlor shock to both shock the newly opened pool and then to maintain the FC level. I'll add that the pool is new to me this season and there may be an issue with the SWG. Prior to 10 days ago the pool looked and test good. I had a report from the pool store tester that had the CYA level at 125 ppm. Though I am not a pool person, from what I could read, this was too high. That sets the story, now my question.

I had an algae bloom, for what ever reason, and I noticed just prior to that my test strips were indcating a lower CYA level. After the bloom those results repeated, both with a strip and with a droplet chemical tester. I began to wonder if pool store #1 needed calibration. So I brought a water sample to store #2 and got a test result of 24 ppm, consistent with my lesser grade at home tests. From what I can read CYA does it naturally change that much that quickly.

I am left wondering if there's a calibration problem or ... could the algae be responsible for the apparent drop in CYA ? Can pro testers be that far off?


Mod Squad
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Jul 16, 2012
Central MD
The "pro" tester is you using the Taylor CYA test and being consistent. Comparing test strips to pool store results is not a productive approach. Nor is using either one individually. Fortunately you are the answer... i.e. testing yourself.

Algae doesn't consume CYA. There is a bacteria that does, but your current issue is one of testing.
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Well-known member
Mar 25, 2019
Houston, TX
Test strips are worthless and the CYA test is a little more trickier than the others. Its a bit subjective to be honest. At levels approaching 100, you should be diluting it by half and doubling your test result to get a better reading


Well-known member
Jun 17, 2019
Central NH
I have some of the CYA readings/test results from store #1.
5/8 - 49 ppm, matched test strip
5/22 - 106 ppm, after much shocking with dichlor so it seemed reasonable if at/above good level
Store #1 said to shock w/31 lbs of dichlor, I think I used cal hypo instead.
5/24 - 118 ppm, store #1 still recommending dichlor as all FC, TC levels are low.
5/31 - 125 ppm, my faith in store #1 faltering
6/5 - <30 ppm !!!!, using test strips and HTH liquid test (dot disappears), what is going on?!?
6/11 - 24 ppm, measured by store #2 (I wasn't a customer), again matching strip and HTH tests that day and previously.

The initial progression of CYA looked reasonable to this newbie's eyes given the use of dichlor. I got concerned with continued dichlor recommendation after the level got past 100 ppm. I got fed up and decided to double check #1 by going to #2, after my at home tests showed a huge difference. While the simple tests at home are subjective, I just couldn't believe they could be that faaaaaar off. Store #2 seems to confirm it though.

Because I don't like writing off the installer and because I had heard of a questionable biological way of reducing CYA, I felt I needed to ask a silly question. If algae can't "eat" CYA then I don't see anyway to square #1 results with the rest, other than their equipment is out of whack. I stop short of accusing them of malfeasance without strong evidence (Hanlons razor).

So with that settled I can now turn to the other myriad of pool issues that have sprung up: questionable SWG, sleeping in-floor cleaner and green pool, all topics for other threads.

Unless someone can add more knowledge, I'll consider this (silly) question answered.

ETA: Having said the above, I just read this in another post ...

If the pool is olive green and or super cloudy or your CYA degraded into ammonia, you will lose FC almost as fast as you can pour the bleach in.

So can algae degrade CYA into ammonia? Is that process, CYA -> ammonia by any means, a way my CYA readings could be so disparate?

And someday I'll split a water sample in two and have both places test the same sample.
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Mod Squad
TFP Expert
LifeTime Supporter
May 19, 2010
Tucson, AZ
Don't trust pool store testing. Don't trust test strips. Period. This has been proven to us repeatedly for 10+ years on the forum.

You need to get one of the recommended test kits if you want to have consistent and understandable test results.
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