Can't hold chlorine, all other readings good

I can't get the chlorine to stay in my pool. 20 x 40 IG. PH = 7.3, ALK = 100, CYA = 35, Hardness = 210. I have 3" pucks in the chlorine feeder and the FC goes to 0 within 12 hours. I had a CYA of 0 last week. I added 8 lb of stabilizer and got the CYA up to 35. I thought that would solve my problem but I still can't get the chlorine to hold. I have been adding 1-2 gal of bleach daily which gets the FC up to 4. Then 12 hours later it's near zero, even with the pucks in the feeder. The pucks have always worked well for me throughout the summer. I just add bleach after a heavy usage to boost chlorine. The pucks cost more but are much easier. The water is crystal clear. I can't figure out this problem.

You need to get a test run the gives you both total chlorine and free chlorine.
This will give you combined chlorine, which if you have more than 0.5 ... you have a problem. Often a good indicator of high CC is that the pool smells strongly of chlorine.

If your pool is not covered and getting a lot of sun you will need to add something like a half gallon of bleach a day to keep the free chlorine up. Add it at sunset and test again first thing in the morning before the sun gets on it.

If you've still got chlorine you're OK and just losing it in the day. Other things like kids, dogs, leaves etc will add to the sun's depletion rate.

If however you find that the chlorine is all gone in the morning then you likely have another problem. A borderline algae bloom maybe. Maybe a lot of crud in your filter that is consuming the chlorine. Have you openned the filter to see what kind of shape the media is in ( sand or cartridge or whatever )?

Find out what your CC (combined chlorine) is and post if for us.

Some ideas that I hope will help.

Thanks for the input. My filter is DE. I changed it when the water was clear after spring clean up (about 10 days ago). The pressure has stayed at 12 ever since so the filter isn't removing much of anything. The water is so clear I can't imagine any algae at all being in the pool. No chlorine smell. I guess I'll have to go to the pool store and get my water tested for CC.

Chip,

Read the post "How chlorine works..." posted this morning.

I read the thread about how chlorine works which made me thnk of another factor that is new in my pool this year. I bought a new solar cover this year. The old one was blue and the new one is clear. Could that be allowing more sunlight to come through and consume more chlorine than I did last year? Maybe my chlorine feeder just won't keep up with the sunlight coming through the clear cover.

I'm a newbie here but from what I've read here goes - Have you retested your CYA? If you got a CYA of 35 up from 0 shortly after adding the 8# of stabilizer, it may be much higher now (don't know your gallonage) and with the use of the pucks that adds to it also. Could that explain the vanishing chlorine?

A higher CYA will just tie up more of the free chlorine but you would still be able to measure it so this is not likely the problem with the vanishing chlorine.

Yes, the clear cover will allow more sunlight through than the blue cover so it will cause more chlorine breakdown than before but it sounds to me like there's more than this that's causing such high chlorine usage. I'm guessing if you're using a solar cover you are not in a hot environment yet.

One test you could try is to add the chlorine after sundown and measure the level. Measure it again in the morning and see where it's at. That should help identify if you're losing it to sunlight or something in the water.

Peter