Please help me!! I have a new 20k gallon salt pool (built 11/07). A month ago, it was dicovered my Pentair chlorine generator was defective and it was replaced. All LED lights indicate the new one is working properly. I have had my water tested AT LEAST weekly at Leslie's Pools and each time, I show absolutely 0 FAC and TAC. I've tried superchlorinating by running the pumps for 24 hours at 100% output. No change! I just shocked the pool this morning with 3 lbs Chlor-Brite (sodium dichloride). About 2 hours after I shocked it, I showed about 3 ppm FAC. Now, 12 hours later, it's all gone (0 chlorine!). I run the pumps a minimum of 10 hours a day from 8a-6p. I add 1 qt Muriatic acid about every 3-5 days and 2 1/2 capfulls Pool Perfect with Phos-free once a week. At my last test (this morning), I showed pH at 7.8, TA at 100, Calcium Hardness at 275, CYA at 90, Phosphates were at 100 ppb and the salt level was 3900. I had the techs test for nitrates this time and I showed 0. My water is chrystal clear, never had algae, but what is happening to my chlorine? Any suggestions?