Fiberglass pools and CYA

Jun 22, 2012
I wish I had found this forum before I Opened the pool this summer!
I have a fiberglass pool and therefore it is not to be drained unless the water is pumped out from underneath (huge hassle), and my cya level is high at 80.
I live in Toronto so the pool is only used 3 months per year and this is my second year. The pool has a copper ionization system and a puck feeder. Due to a bylaw here a saltwater pool would have cost a crazy amount given the sewer access at my house (don't ask the bylaw is too strange to explain).
My question is how much can I lower the CYA levels by exchanging a several inches of water each week? Is liquid chlorine safe for fiberglass pools?
Pool is clear, I just shocked the pool to re-establish chlorine after these test results
Lab results: tc .6 and fc .3
CYA 80
TA 110
Ca 125
pH 7.3


TFP Expert
LifeTime Supporter
Feb 23, 2008
Denton, TX
Welcome to TFP!

If you stop using tablets and go to bleach, you can live with a high CYA like that. By chlorinating using the cholrine/CYA chart at pool-school/chlorine_cya_chart_shock, you will know just how much FC you need to operate on a day to day basis.

You can lower your CYA by replacing water a few inches at a time. It just takes a long time.

With a CYA of 80, your absolute minimum FC should be 6 ppm. You are going to have an algae outbreak soon if you don't raise your FC. You also need to consider getting a test kit that has the FAS/DPD chlorine test as this allows you to test for FC levels above 5 ppm, essential for shocking. Please consider not using your copper ion system as well. Copper is what stains hair and pool surfaces green. There is no need for an ion system when your pool is properly chlorinated.


Mod Squad
TFP Expert
Platinum Supporter
LifeTime Supporter
In The Industry
Apr 1, 2007
Sebring, Florida
My question is how much can I lower the CYA levels by exchanging a several inches of water each week?
AS much as you want. If you drain/refill10% of your water you will have 10% less CYA, etc.

You can do it simultaneously quite easily. Pump off. Assuming your fill water is colder than the pool, start refilling by sticking the hose in the bottom of the deepest point. Then start siphoning from the opposite end of the pool and only go maybe a foot below the surface. You could lover your CYA down to a manageable 40 or so in very short order and never change the level of the pool water.
Jun 22, 2012
Thank you for your responses, the problem seems to be getting a lot better (I think). After having lots of canon balls and replacing some water the latest tests are as follows:
Tc: 1.6
Fc: 1.3
pH: 7.5
CYA: 70
Ta: 100
Now I just have to learn to use liquid chlorine (a slightly daunting task). Wish me luck
Fiberglass pool 11x24 in Toronto


LifeTime Supporter
Good job so far, but you need to get that chlorine up. At least in the 3-5 ppm range. Without a margin of free chlorine, you run the risk of a storm, or even a really sunny day, putting you at a chlorine deficit, and then you get to go though the shock process. Use the pool calculator, and get some 6% bleach.


Well-known member
Oct 12, 2009
With a CYA level of 70 or 80, it shouldn't take a long time to lower to 40-50. If it were me, I would just go cold turkey on the stabilized chlorine and let the level fall slowly on it's own as water evaporates, splashes out etc and is replaced.