Where has my CYA gone?


Well-known member
Nov 15, 2018
Perth, Western Australia
FC 9
CC 0
TA 90
CH 290
pH 7.65
Salt 4700
B 55

The basic chemistry hasn't changed except I added borates about a month ago.

I did an initial test on 14th of Nov and CYA was ~40. Dosed it up to 70. Re-tested 27th of Nov to confirm. Pool store reading was the same (so 70. They are good for a cross check).
Periodic tests since then held relatively stable. Noticed ~13th Feb it was down to 60.

Re-test yesterday and it's < 30.
Check CYA reagent with 50ppm standard and it's "about right". Not convinced so I mixed up a 70ppm standard using fresh granular CYA. Re-test pool and both standards tonight.
Same result.

So sometime in the last 4 odd weeks something has eaten a pile of stabiliser. Colour me quite perplexed.
I tested the water about 4 times using different bottles of reagent, and both at pool (23C) and room temperature (30C).

So I'll dose it back up over the next couple of days, but this is most peculiar. We've had nothing out of the ordinary. I did hit the pool with a pile of Cal-Hypo to knock out what looked like Mustard Algae living in the bottom of the pool which pushed the FC up to about 28, but it was passing OCLT both before and after.

Salt level has risen, so it's not a water dilution issue.


Well-known member
Nov 15, 2018
Perth, Western Australia
I was going to suggest a possible leak but your salt hasn't dropped at all either right?
Nope. I check that pretty frequently. Top ups are only ~600-1000L per week, which is about right for the weather and surface area.

It has me baffled. The other thing that is odd is the chlorine demand really hasn't risen. It's still ~2.5-3ppm/day. I'd have expected if the CYA dropped from 60 down to > 30 I'd see more chlorine burn off during the day.

Oh well, I've just put 25ppm worth of granules in. I'll let that dissolve over the next couple of days and re-test. I might take a sample to the pool shop. Their readings always match mine for CYA.


Well-known member
Jul 7, 2018
Pittsburgh / PA
I've not been on the forums for a few months... but CYA will naturally dissipate if I am not mistaken. Meaning even a closed pool with a cover on it will see CYA levels degrade over time.