low CYA, high chlorine...unsafe?


May 26, 2014
Richmond, VA
I have finished a complete drain and refill recently due to metal staining and high CYA. After some discussion my husband wanted to use the trichlor pucks to gradually raise the CYA instead of adding it separately. (We have leftover trichlor pucks from before the drain/refill) Using the pool calculator I figured that adding 8 pucks would get us up to around 15ppm (The TF-100 was showing the CYA as zero) even though it would raise the FC significantly. I was tired of having to add bleach constantly because the FC was dropping back down every day. So I added all 8 pucks a few days ago and now the FC is 7 and the CYA (near as I can tell) has gone up slightly. Since the mixed solution is not completely clear like it was initially I'm guessing my CYA to be around 5-10 at this point. (I can still see the black dot but it's barely visible with the tube entirely filled)

I see on the CYA/Chlorine chart that I've way exceed the "max" level. What does that mean? It's toxic and unsafe right now? How does CYA affect that? I'm asking because I've read elsewhere that in some states public pools are allowed to stay open as long as the FC is 10 or less. So 7 doesn't seem hugely excessive unless I look at the CYA chart. Am I at risk of damaging something in my pool? We have guests coming tomorrow evening so I'm thinking I'll take the pucks (what's left of them) out of the skimmer and seeing where our level is at by noon or so.

Thank you in advance for your help.


Mod Squad
TFP Expert
LifeTime Supporter
Jul 10, 2009
Houston, Texas
You need to keep your FC in ratio to the CYA. The CYA is what buffers the FC so it is safe at higher levels. A pool is safe to swim in up to shock levels. Shock level for a 0-20ppm of CYA is 10ppm FC. As long as the FC is 10 or less it shouldn't harm anything. It would be better to add the CYA separately so you don't raise the FC too high.


Well-known member
Jun 1, 2012
If you are using pucks, the CYA is there. It takes time to show up on tests. We have swum past shock level before and nobody even noticed. It is not recommended, but we are still here...A large swimming group is going to lower that FC pretty fast anyway.


LifeTime Supporter
Oct 17, 2014
Spring, TX
You're fine to be swimming with 7.
Give it a day or so and it will be back down at 3 or 4.
Your CYA is still low enough that you will still lose FC fairly quickly.
CYA is Stabilizer, it helps protect the Chlorine in your water from being burned off by the sun. It also reduces it's effective strength.
Without CYA every sunny day will drastically drop your FC lavel.
So you want some, but not too much.
You experienced too much and had to drain your pool because of it.
Too much and the effectiveness of the FC is compromised so much that you have to have really high FC levels and if you have to SLAM the FC level is HUGE.


TFP Expert
LifeTime Supporter
Jun 5, 2012
DFW, Texas
First of all, relax. An FC of 7 ppm @ a CYA of ~ 10 ppm is not "toxic" nor will it cause damage to your pool in this short amount of time. Plus, since it is summer, the FC consumption will be increased due to the sun and warm water temps.

With that said, you need to get more CYA in the water for longer-term management of the pool. It would be better and faster to add granular CYA using the sock-in-the-skimmer method and chlorinate separately using bleach. Solid CYA should be placed in a sock, and the sock put in the skimmer basket. After adding solid CYA you should leave the pump running for 24 hours and not backwash/clean the filter for a week. It can take up to a week for granular CYA to fully register on the test, so it is best not to test the CYA level until one week after adding some. However, after 24 hours of adding the CYA, increase the FC target to reflect the starting CYA level + the amount of CYA you added. In addition to the [FC/CYA][/FC/CYA], you may find Pool Math to be helpful.


May 26, 2014
Richmond, VA
I can't find that....sounds like we may need to clarify that a bit....can you help me find it?
Dave, I got the information from this thread from 2007.
Chlorine/CYA Chart

I can see the link to the simplified chart but that one only starts at 20 (for the CYA). So going off the info in the thread it looks like with a CYA of zero (I now realize mine is a little higher than that) then a FC of 7 is alot higher than the recommended shock level. Also on the pool calculator page under "about suggested levels" it states that if your CYA is zero then you should not go over a FC of 5 but it doesn't say why so I freaked out a bit.

Thank you for the quick responses! I took the pucks out and half of them had dissolved already so at least there is some CYA in there. I'm going to stop using them once the CYA is 20 or so.

- - - Updated - - -

I should mention I found the thread from 2007 after doing a google search for "low CYA high chlorine"

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA
Yes, 7 ppm FC with no CYA in the water at all would be quite high in active chlorine level. It wouldn't call it unsafe in the sense that it would cause immediate harm, but it would not be pleasant and over 100 times higher than the minimum FC/CYA ratio we recommend on this forum. So one day in such a pool would oxidize your swimsuits, skin and hair similar to swimming every day for over 3 months. It could also sting your eyes.

The shock/SLAM level, by comparison, is around 10 times higher than the minimum so is more tolerable and why we use that as a practical upper limit.