Here a new wrinkle in this problem. I tested the water on top of my winter cover and the pH is 7.9! So the water in my pool (under the winter cover) was 6.4 and the water on top of my winter cover is 7.9. The water on top of my cover is fairly clear even thought there are leaves under the water at the bottom of the cover.

After the discussions below, I was thinking the pH of the water on top of my cover would be low and hence leached into the pool water underneath. Obviously this is not the case. This problem began only a couple years ago. Prior to that, I never had this problem. I am still treating my pool water the same (bleach for chlorination), it's the same cover. I'm still perplexed as to why my pH would drop so low over the winter but during the summer be steady as a rock.

Could algae cause this? It seems like last year I had what appeared to be black algae on the bottom of my pool when I opened last year. It was a ring a couple feet wide around the circumference of the pool. Wiping with a Miracle Eraser easily removed it and I did not think about it again. I have not looked this year as I have not removed the cover yet. But can black algae cause the pH to drop? That's the only change I have noted over the past couple years.

I appreciate your thoughts.

Jim