I'm obviously doing something wrong. I brought the pool to shock before closing in the fall, but every spring I open the cover to this. It also smelt like a horrible fart. When I got the pump running all the kids ran for their lives. Thankfully no one puked.
In our climate it's almost impossible to lift the cover back and add chemicals mid-winter. The pool was still a block of ice on top in March.
I'm still in process of "S." and have not quite gotten to L.A.M. this year. (Baseball, full time job, three kids, hubby broke his arm so he's no help)