No Chlorine after Shock

[HenderPool]

Hello,

I have a 20,000 gallon in ground pool with a vinyl liner. Earlier this week, I noticed that my chlorine levels were consistently 0 after adding liquid bleach 24-48 hours earlier. I decided I needed to SLAM it, so I added 8 lbs of 73% cal-hypo last night, per instructions from the pool math app. I tested this morning, and the chlorine was 5ppm around 9 am, and 0 by 1pm.

I went to the pool store and they suggested 50 lbs of shock, which is nearly $300, and seems absurd. What do you all suggest?

Other readings:
Ph: 7.6
Alkalinity: 90
Calcium Hardness: 300
CYA: 0 (this has been high, ~100, since I bought the pool because a previous owner used tabs; this is the first time I have ever seen it be low).

 

[mknauss]

How are you testing your pool water chemistry?
You need a CYA level of 30 ppm to best follow the SLAM Process. Adding chlorine once is not following the process.

 

[HenderPool]

How are you testing your pool water chemistry?
You need a CYA level of 30 ppm to best follow the SLAM Process. Adding chlorine once is not following the process.

I've been testing with TF-100, except for the CYA, which I'm using strips for right now as I'm waiting for replacements to arrive in the mail. Strips and pool store both showed 0 for stabilizer, though I get that you'd prefer to see the result from the TF-100. That'll be a few days though.

Yes I see the process requires keeping the chlorine level high throughout the SLAM process. I can implement that if that's all that's recommended, but likely not until next week as we are traveling in there interim days and it seems like a waste of money if I can't keep it high for as long as needed.

 

[mknauss]

Once you get our reagent run a CYA test. Then add as necessary to get to 30-40 ppm.

Follow the SLAM Process when possible. Add at least 5 ppm FC per day, if possible, to keep algae from growing exponentially.