The snails and mussels may not have enough
food to survive, also mussels like moving water.

I had two mussels that were put in last year.

On cleaning out the small pond this week, I found one of the shells wide
open.
When I lifted it from the water, the mussel just slid out of the shell.
I assume it had died, as it was no longer attatched ?

Funnily enough, that was the one nearest the return pipe.
The other one so far seems fine.

Peter