Just wanted to post a word of caution about something I just ran into today, since I haven't seen something quite like this before during many years of keeping large shells in tanks with marine fish. I'm breaking down some of my tanks in preparation for a move, which involves rehoming some fish. This afternoon, a Bangai cardinal of mine, which is one of a few fish that have to go before the move, saw me get the net out and then thought it would be neat to slowly swim into a large empty whelk shell that was there for some large Clibanarius hermits in the same tank. He carefully and decisively went into it and then just didn't come out. It wasn't the sort of panicked, split-second dash that I've seen result in stuck fish before - it was a slow and careful entry into something with no exit and numerous other potential hiding spots nearby. After a while, there was a lot of thrashing about and wobbling the shell and I realized the cardinal must have wedged itself inside and needed intervention. I recently got some bone cutters for breaking up live rock and coral skeletons, and those made the job pretty easy (not sure how I would have been able to do it otherwise - good tool to have on hand!). I was able to clip away the shell one piece at a time without harming the fish and saw that he really had gone in as far as he'd fit, which bent him around in a way that made it impossible to back up. So, now I know be careful with dumb fish and large spiral passages to nowhere. Some other shells with larger, rounder apertures probably wouldn't have had this issue. The offending shell was one with fairly narrow aperture with one long spindly side.
