shells

Question
hello,
I was just wondering if it was normal for a crab to change shells every night. I have three crabs and the largest one, which is still small but is leaning to towards medium changes shell every night and last night I saw him guarding five shells from my smallest crab who was trying to change shells. Once the smallest crab finally was able to change from his old shell the bigger one snatched his old shell up before he was even finished changing and then started snapping at the smaller crab. I don't know if i should buy more shells because he is not comfortable in any of them for any length of time. I have 20 shells in the tank all sized between 1/2 inch opening to 1 inch. I also clean them every day with salt water. So I am kinda stumped, are crabs usually territorial over shells that there not even in? I mean this medium guy changes shells with my hand in the tank. anyway any insight you could give me would be great thanks,

Answer
If he hasn't picked a shell to stay in for awhile I would rotate some shells out and add new ones in. Offer him some bigger shells with different openings. Make sure the shells are clean inside, and there's nothing in there irritating him.

I've had a crab hold onto a shell, try a different one on and not let any other crab near either shells.