If you keep shellies and you have one that’s shell bound, there are several possible reasons. Fish use shells normally for two primary purposes – shelter and egg laying. If you have a fish that stays confined to a shell and it’s not a mature fish, you can rule out the latter.
Shelter seeking comes in two varieties – shelter from aggression and shelter for illness. A shellie being harassed will naturally seek an empty shell to avoid the aggression, but a sick or injured fish will also utilize a shell and will often go there to die.
If you have a fish that stays just inside the shell but faces outward, it’s most likely protecting eggs or fry. If there are fry, you’ll usually see them. You may also see the eggs. Many shellie species lay their eggs just inside the aperture.
Watch your shell bound fish and see how it behaves in or near the shell. If aggression is the cause, it will typically make regular attempts to back out to see if the aggressor or threat has disappeared. If the fish remains in the shell for long periods when no other fish are nearby and you’re certain it’s not protecting fry or eggs, there’s a high probability it’s sick or injured.
Learning and understanding the behavior of your fish is the best way to determine when your intervention is needed.