We finished searching. The seashell is gone. I realized the night I cleaned the tank was three nights before we restarted it...and the garbage did go out within that time.
I picked that shell up on the beach when I was in high school. It was soothing to hold. I'll never hold it again.
I hope some abandoned puppy at the dump finds your seashell and they become friends for life.
@amarand It's almost exactly like the shell in the third picture here. Just more of it worn away. Thank you for offering puppies.