Your local farm or feed supply store can help you with the correct products for worming and treating for coccidia. It's fairly straightforward.
Soft, thin-shelled or shell-less eggs can be symptomatic of a problem in their reproductive tract. Earlier in their lives they may have been exposed to a respiratory virus that gravitates to the reproductive system causing damage that can manifest as thin, soft or even completely missing shells which, in turn, encourage the chickens to eat the easily accessible eggs. The symptoms of this kind of respiratory infection can be very slight, yet the damage to their egg laying organs severe.
I would suggest, worming them, treating them for coccidia, then following up with thirty days of a vitamin supplement. Hopefully getting rid of the parasite load will help them recover their egg production capacity, but unfortunately, the damage may actually be permanent. Egg eating is a bad habit and can be tough to correct, so if you can get the egg shells to harden, and they still break and eat them, then starting over with some new birds would probably be the most practical solution for you. If you have more questions, please ask, I'll be happy to answer them for you.
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