Disappearing CYA

[bsjeep]

OK, so I added CYA in say March, then when returning from vacation around early July realized it was zero, so I did a bunch of stuff, the pool was setup pretty well.

CYA was 70-80, SWG, TA 90, CH 300-350

Now about 40 days later, I'm at:
CYA <20 maybe zero
TA 120
CH ~400
FC 1 or less

Now in this time, the pool is 5ft deep, there's a spa, the water has been 90+ deg in the pool. There is a safety pool cover.

The SWG had problems keeping up with heat/sun/ and it was in desperate need of cleaning which I just got done last week. So the 40 days was probably FC 1 at best as it just couldn't keep up. This is SWG at 100% I'm assuming now that it's cleaned it will be different. Mind you the pool looks great, clear, blue, etc. non-chlorine shock used every 1-2 weeks.

The PH fluctuates as I assume now all SWG pools do, I try and keep it at 7.4-7.5 but it constantly climbs.


I'm wondering what if any factors would cause the CYA to disappear as it does seem strange from what I've read.

 

[chem geek]

That is strange over only 40 days. If the FC got to near zero, then bacteria could convert the CYA to nitrogen gas or nitrate (if you are unlucky) or to ammonia (if you are unlucky). Could there have been periods of time when the SWG was off that the FC got to zero?

 

[bsjeep]

Could there have been periods of time when the SWG was off that the FC got to zero?

Yes, the SWG was extremely clogged and it's performance was going down so FC was 1 and then probably 0.5 for a while and probably close to zero the last week until I figured out to clean and acid bath the SWG.

So the FC of the pool might of been near zero, although the water in the pool return had some level of chlorine in it (like if I took a sample of a pool return jet).

So are we saying if I do nothing to my pool first the FC zeros out, then the CYA would zero out, then the pool would turn nasty?

Should I be concerned about these bad by-products you mention like ammonia?

 

[chem geek]

So the degradation of CYA by bacteria takes only a few days so that's probably what happened, but you got lucky in that such degradation must not have resulted in much ammonia or else you would have seen a huge chlorine demand well beyond what the SWG could handle. A drop in CYA of 60 ppm could, worst case, result in ammonia requiring around 150 ppm FC to get rid of.