I would use sand as substrate.
As far as the shells whale eye shells, escargot shells and turbo snail shells work well.
Here's the shells I used, they are turbo snail shells I got from the LFS.
Escargot shells
Whale eye shells
As far as the shells whale eye shells, escargot shells and turbo snail shells work well.
Here's the shells I used, they are turbo snail shells I got from the LFS.
Escargot shells
Whale eye shells