Was told that snails could live with Opaes, but the snails don't seem to be doing well

Hi there I need some advice. I was doing research on opae ulas and snails and everywhere I checked said that netrites could live with them in their brackish environment. So I bought a few and when I asked the shop keeper they said they could too. I bought a variety of them, horned snails, Pumpkin snails and black helmets snails. But when I put them into the tank they immediately freeze up and started shrinking into their shells. I thought oh maybe theyre just scared but a week later and they were still there. I picked them up and they were all closed, open ajar, or the Pumpkin snail which snail flap literally just fell out and I think it's basically dead. Idk what happened. I thought they could live in brackish water just fine. Any advice on what's going on?