HCA: Hermit Crab Association

I have a crab, Thunder, who's been in the same shell for almost a year now (it was to small to begain with), and is so big for it he can't even get his eyes inside. He's and extra large in what I'd say is a medium shell. I have three extra large shells and 4 jumbo shells in there but he won't touch them. He's done some carving of the shell (forget what it's called when they shape it to themselves) and our jumbo is pretty protective of the larger shells but that's all I can think of that would stop him. I've used some of the techniques for naked crabs (the shallow water with some other options) but he's just stubborn. I'm worried mostly cause he has a bit of an attitude and likes to push other crabs away from the food and water and sometimes even guards the hides and with his eyes and a bit of his abdomen exposed I'm worried he'll pick a fight with a bigger crab and get himself hurt. Just looking for any tips to try to get him to switch.

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what species is he? and what types of shells are you offering? he may have a particular taste in shells, who knows?