Quote Originally Posted by F3AFlyer7
Hmm...


See I originally thought it was the CYA also. However, these are all happening on pools that are being opened for the first time this season. We check their stabilizer every year because we drain our pools half in the winter. They all have very low stabilizer from the loss of water.
BINGO! Closed pools can lose CYA by biodegrationl. The lowered levels are not completely from the loss of water. A 50% drain will only remove 50% of the CYA. I will bet money that your levels are lower than that after refilling! Anerobic bacteria will eat the CYA and excrete large amounts of ammonia and urea. Symptoms are high TC and FC not holding until the ammonia compounds are burnt off. Not a recognized phenomenom but one that has had much discussion in this forum and there is research that backs it up. Don't have time to find some of the other threads on this right, have to get to work very soon, but there are several with links to some of the science behind it. You need to compare the CYA level on closing with the level on opening BEFORE you refill. If the CYA has dropped that is the problem.