I was trying to show my brother why he can't keep chlorine in his pool and hit a brickwall. He has been using shock and tabs for two years. I expected his cya to be off the charts. I ran CYA and it shows 0. This pool is 8400 gallon vinyl, above ground pool. I can understand now why he has no chlorine. But why does he have no CYA? He said he threw 3 bags of shock in last week and still no CYA.