Loosing all CYA over winter WITH a good FC in the spring

[pasterisaj]

Installed the pool in August two years ago (long story but did it myself, big learning experience)

FC - maintained between 3-5
PH - ~7.5 (had a lot of trouble holding it)
TA - very low
CH - very low
CYA - 35

Close the pool the first year with a "pool closing kit" - Opened up to a green swamp, no CYA
After cleaning it up, I raised the TA to ~80 with baking soda and the CH was corrected using calcium shock.

Raised the CYA to 45 this time as my sun loss seemed to be high.
No more PH instability after adjusting TA with the baking soda.

Pool was sparkling with several comments asking how I did it and one 'who's your pool guy?'

Closed it late (water temp ~50°) with only 12.5% bleach at FC = 12.
This spring I opened the pool to clear water (except what junk fell in from taking off the cover) with a FC = 2.

I had run out of reagents to test CYA so just assumed that with no algea bloom I would be fine

Just tested last night with my nice new TF-100 kit to find no CYA - perfectly clear after mixing for a minute.

PH is stable at 7.5
FC was 3 by both OTO and drop testing
CC - zero

Where does my CYA go? I do drain 3"-4" several times during the season from rain but definately not enough to zero out the CYA.

Other than using the closing kit the first year, the pool has never seen anything other than 12.5% bleach, baking soda, and PH +/- adjusters (only before getting TA correct).

Is there something else I should be doing to combat this CYA loss?

Aaron

 

[JasonLion]

The chlorine probably came in with the water used to refill the pool. The CYA vanished over the winter due to bacteria growing in the water while FC was zero, the ammonia outgassed, and the fill water chlorine was enough to restore the FC level.

 

[pasterisaj]

This year I didn't add any water to the pool when opening and still had FC in the pool when I checked with the cover still on. Also, my tap water doesn't show any FC.

The only thing I can think of is that the OTO test was actually showing CC in the spring but wouldn't the pool be a nasty mess with that much CC and no FC?

One other theory is that my neighbor had a lot of loam delivered and I had to clean dirt off the bottom every day for a week - chlorine never went to zero but I was using a OTO kit.... maybe the dirt brought in a lot of CYA hungry bacteria (was loam from a farm)

(EDIT) Just reread my posts - I keep assuming the reading from the OTO kit is FC

 

[JasonLion]

We are not currently aware of any way CYA can go down while the FC level is above zero. without water replacement. You can have CC and still have clear beautiful water, though that is not at all common. There are several forms of CC, some more annoyingly obvious than others.