Oyster shells, the answer to all of your weak shell problems.
"Thin egg shells are observed when calcium, phosphorus and vitamin D3 are not provided in diets at adequate levels. It is more often observed during periods of hot weather because calcium is conserved and retained within the hen's body less efficiently."
- Mississippi Agricultural and Forestry Experiment Station
I had a bag of oyster shell gravel that I occasionally mixed in with the pellet so I mixed some more in. I was under the impression that the shells aided in digestion, but I guess they are more of a calcium supplement. So far so good.