I found several surface chips on the waterline ceramic tile after I removed the winter cover. This is the 2nd winter, the first winter was OK. I did eveyrthing the same closing the pool. The only difference was this time I placed a cover pump on top of the winter cover, so I did not get much ice formation on top of the cover, but I did get very thick ice, 8 to 10 inches under the cover. The water level at closing was about 2 inches below the tiles. The chips are of difference sizes, from about 2-3 mm to 1 cm in diameter, irregularly shaped, all very thin, I manage to find most of them at the pool bottom and I glued them back using marine epoxy. Other missing areas I just used some touchup paint. The repair turned out good, you can't notice it unless you look very closely. The tiles were supplied by the pool manufacturer, San Juan Fiberglass Pools, and the PB did a good job using special epoxy as specified by San Juan. I am kind of puzzled why this happened, is it possible that the moisture inside the tiles causing freeze damage? Or is it due to the pressure from the pool water freezing and expansion? Maybe I should not have used the cover pump, because it didn't happen after the 1st winter. Any other suggestions?