Mystery Snail Has Bad Shell

I was checking out my snails and I found out that one of them had his shell broken at several places or looks like eaten what is it. how to prevent shell from dissolving. I worried for him I have total 5 mystery snails (apple snails).

 

Calcium, calcium, calcium!! Also what else is in your tank? Can I see a picture?

 

Calcium, calcium, calcium!! Also what else is in your tank? Can I see a picture?

I don't have a tank for him right. I just sold the tank 2 days ago and will be getting a new one in a day or two right now he is in a bucket with 3 other snails. how do I increase calcium in the water? (also note that we don't get any sort of meds for fishes here in my area except for fin rot ich) is there any home made way ro do it? please suggest me.

 

ZuchinnI or a bird cuttle bone should help

 

Your water might be too acidic as well. I'd also use something like crushed coral as a substrate in the tank.