Missing CYA

[C3Cl3N3O3]

Hello gang,

I've been a lurker here for a year or so. I switched to TFPC/BBB late in the season last year after struggling with my pool all year. I purchased a Taylor K-2006 and found my CYA last year to be 200 or more. I did a series of partial water changes on my 20000 gal vinyl pool with city water and got it down to 100. I covered the pool over the winter and opened it up a few days ago. The water temp was already to 70 here in SC and there was some blue-green algae color, but not severe (the cover has a few holes in it). My numbers aren't all that bad:

pH: 7.2
ALK: 70
CYA: 40
FC: 9

The mystery is the low CYA. I dropped the level by 5000 gallons during the close, if that, and just refilled. I was expecting a number closer to 75. I know these things can't be predicted exactly, but considering how hard it was to drop the CYA last year, I'm curious where that substantial delta could have magically gone. 40 isn't a bad number, much better than a high number, but my pool gets a lot of direct sunlight so it may be on the low side now.

 

[BoDarville]

Welcome to TFP!

If we rule out a leak, it's possible that you could have some ammonia in the pool which is caused by bacteria that consume CYA and convert it to ammonia. Usually when this happens, CYA goes to zero. However, you could have early stages of this going on and caught it before the CYA went to zero. One way to determine if this is the cause is to see how your FC holding up. If you are not sure, I would suggest an Overnight Chlorine Loss Test.

Also what is your CC level? Ammonia will cause high CC levels.

 

[C3Cl3N3O3]

I don't think there's a leak, the pool has always held water well.

I've been keeping it at shock level until it (hopefully) clears up. I just tested it at dusk at the FC is now at 12 ppm. The CC barely registers anything, .5 ppm if that (probably zero). I'll test again at dawn. Thanks for the reply!

 

[BoDarville]

If the CC is 0.5 ppm, then I think we can rule out an ammonia problem. Curious to see what your FC reading is in the morning.

 

[C3Cl3N3O3]

Well I checked it this morning after sunrise and the FC held right at 12. No loss at all. So any bacteria that may have been there seem to be gone now.

 

[pooldv]

Ok, back to our regularly scheduled SLAM - Shock Level And Maintain

 

[tim5055]

Well I checked it this morning after sunrise and the FC held right at 12. No loss at all. So any bacteria that may have been there seem to be gone now.

Well, CC less than .5, passed OCLT.

Water clear/any visible algae?

 

[C3Cl3N3O3]

When I opened it on Saturday it was cloudy and bluish green and there were a LOT of particles that had to be filtered, but today it's fairly clear.

 

[tim5055]

When I opened it on Saturday it was cloudy and bluish green and there were a LOT of particles that had to be filtered, but today it's fairly clear.

I would probably keep it at shock level (what we call a SLAM) until it clears up. Give it a good brushing each day to make sure no algae is hiding anywhere.