CYA more than DOUBLED after shocking???!!!


New member
Jul 23, 2018
Frankfort, KY
Our CYA readings have always been normal at 30-50ppm. We use Trichlor tabs in chlorine feeder. Just 3 days ago we shocked with the Clorox 4 in 1 formula (sodium dichlor). Big mistake. Now my CYA readings are OFF THE CHARTS at 150. HOW?! Could my test strips be wrong? According to the calculator on this site CYA should have only gone up about 6ppm. What happened?! We used one pound of the shock treatment, and our pool is about 11,000 gallons.

I've since turned chlorine feeder off completely to stop addition of even more CYA. FC and TC are staying high around 8-10ppm thanks to all this CYA and no sunlight at all the last two days.

I do only use the Aquachek pool testing strips. Getting a decent FAS-DPD testing kit via amazon on Wednesday.

I guess my question is: is it possible to raise CYA from 40 to over 150 in just one shock treatment? How does this happen?

Before shock: CYA = 30-50ppm
After shock: CYA is off the charts. test strip looks like 150 but i know anything over 100 can't be measured accurately


Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
Laughlin, NV
Welcome to the forum! :handshake:

Test Strips simply do not provide the accuracy and consistency you need. Because of this we do not make chemical recommendations based on them. You need to order a proper test kit.

Order a TF100 test kit
The only other real option for a test kit is a Taylor K-2006-C. Be careful comparing prices because the K-2006 comes in sizes, designated by a letter. The basic K-2006 has .75oz bottles. You need to get the K-2006-C to get the larger bottles that you want. Even then it is a little short on the reagent & powder for the FAS/DPD test.

I also have the SpeedStir. It makes testing much easier.

I suggest you read Pool School - ABCs of Pool Water Chemistry and consider reviewing the entire Trouble Free Pool School book.

You may also want to get the Pool Math app.


New member
Jul 23, 2018
Frankfort, KY
Thank you. I have read the entire Pool School book cover to cover and have read countless posts in the forums. Got to get a good testing kit and I guess I can tackle my problem then. I wish I had seen this site before. I will be taking a sample of our water to be professionally tested tomorrow.


Well-known member
Jul 7, 2018
Pittsburgh / PA
My guess is your CYA was already well over 40 if you were regularly using tablets in that pool. Those test strips were never accurate.

Stop using test strips, and stop using the chlorine pucks (dichlor or trichlor). Start using regular household bleach, or the pool chlorinating liquids.

Sounds like you have a good test kit coming. Step 1 is to get a good CYA test result. With a CYA that is likely over 100 already, you'll need to do a "diluted" test.


Those pucks and the powdered "shock" both contain CYA. CYA is additive meaning each time you use a puck, or a bag of powdered "shock"... your adding more CYA to the water. Unless you drain and refill the pool, CYA levels normally don't come down very much at all.

This is why you use bleach to chlorinate the pool. It has no CYA in it.

Also, any product with the name "shock" on it is designed to take your money. You don't need a product with the word "shock", or "extreme" in it to perform a "shock".

Shocking your pool is a process, not a product. You can shock your pool with regular household bleach. You can shock your pool with those trichlor tablets. You can shock your pool with Cal-Hypo powder.

Shocking is simply rapidly bringing your FC levels up to a much higher than normal level, and keeping them there for a period of time. Not some magic in a bag powder.


Well-known member
Sep 27, 2016
Oshawa, ON, Canada
Just as a comparison I have a few cruddy test strips that I pull out for a gag every now and then. Two weeks after opening I dipped a strip for gags to find it telling me I had zero CYA. Reality: I had added 40ppm worth the day we opened. The Taylor cya test confirmed 40ppm in the pool.

Today was my recheck day. All the rain and backwashing has dropped my cya to below 30 according to the Taylor test. For kicks I dipped a strip: now it's telling me I have 30-50ppm...There's no consistency unfortunately.

Oh, and PH read normal on the strip, but was actually getting low due to the 4" of rain in the last two days... boosted that this evening. CYA is left for tomorrow as the pump is sleeping [emoji18]