So I go to pool store about every 3-4 weeks just to double check myself ( probably first mistake ). About 3 weeks ago CYA was at 40ppm ( PS reading ) so I added enough to get to 50 ( hot Houston ). Yesterday went to PS and it read "0" ! PS guy asked if I had drained and refilled water ( no ). Asked if we had bad splash out ( no ). Asked how long I let pump circulate before getting sample ( 1 1/2 hrs like previuos ). He said I should let pump run 4 hrs prior to getting sample. I also said we have had alot of rain ( daily ) and my overflow might be causing loss of water. He said probably not since CYA tends to settle at bottom of pool and that is why the 4 hours of pump prior to sample. I have not read that before. Anyone know if this is true or not? Going to restest tomorrow