Hmmmm I think I won't. I'd be scared there is some salt dissolved into the water and other stuff, especially if you collected it on the beach somewhere. If it is a bought one from some shop ( not aquarium shop ) and it is sort of glazed I'd be scared again because I wouldn't know how safe it is.

If it is one bought in the aquarium shop, I'd ask the staff there if it is suitable for freshwater. The other thing is that shells are rich in calcium, it would raise your hardness and perhaps the pH too and buffer the water. This are things I still need to get my head around it for a better understanding how everything connects together.

Thanks for the heads up! I had wondered about any kind of coral pieces or shells before since a few have shown up in the big bag of gravel I bought from the pet store. Guess the manufacturer deals with both salt and fresh water substrates and a few pieces ended up in my bag.

There are usually some imitation shells sold in pet shops Usually with rounder edges too so the fish won't hurt themselves So if you want a sea shell, you can just buy one!
I was thinking about getting a few small ones myself at some point, but it just didn't happen xD

Rather avoid putting any substrate in the hospital tank, at most you can add a few plastic plants and maybe some plastic pots for the fish to hide in.
Hospital tanks should be as simple as possible, and sea shells may be sharp. A sick fish may rest on the bottom and get hurt.

hey i know i am probably too late to matter on this post but my mom put a conch shell in her tank and it slowly bleached out and started to disintigrate. ruinied the beautiful shell she had gotten on her honeymoon with my dad and even worse i was not happy when i cleaned her tank and saw it because no telling what it released into tank.