Zero CYA


Well-known member
Jun 24, 2008
On the pool calculator it states Be careful if your CYA is really and truly zero in the suggested FC area. Why does that matter when adding bleach. Over the winter all I did was added bleach here and there not bothering with CYA but now it is warming up I checked my CYA and it is Zero. I forgot and added 182 ounces of 6% earlier befor adding CYA. Did I do something bad?


TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
Silver Spring, MD
If you have ever added any CYA before, your CYA level will not be really and truly zero. The only time CYA is really and truly zero is right after it is filled and in indoor pools that have never used CYA.

When CYA is really and truly zero, you don't normally want to raise the FC level above 5 even when shocking (thought there are a few exceptions).


May 26, 2013
I am looking for the "why"

What bad happens if one raises the FC above 5 when CYA is 0.

I have an inground fiberglass 20000 gal pool, with a sand filter


Mod Squad
TFP Expert
Platinum Supporter
LifeTime Supporter
In The Industry
Apr 1, 2007
Sebring, Florida
Without the buffering effect of CYA, chlorine can be pretty aggressive, doing damage to swimsuits, corroding metal parts of pool equipment, etc.

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA
tdkorson, Welcome to TFP! :wave:

5 ppm Free Chlorine (FC) with no Cyanuric Acid (CYA) in the water has an active chlorine (hypochlorous acid) level that is nearly 80 times higher than the minimum levels in the Chlorine / CYA Chart, 8 times higher than the shock levels in that chart, and over 3 times higher than the yellow/mustard algae shock level which is the highest we ever use and only for unusual circumstances.

The reaction rate of chlorine with swimsuits, skin and hair, reacting with metals for corrosion, outgassing from the water, creation of disinfection by-products, etc. are all proportional to the active chlorine concentration.

Many commercial/public indoor pools have no CYA and yet even with 1-2 ppm FC their active chlorine level is at least 15 times higher than the minimum FC we use in our residential pools. This is one reason why such public pools can be unpleasant with faster degradation of swimsuits, flakier skin and frizzier hair.