Looking over the other hermits, I realized that at least one was in a shell a couple of sizes too small. I dug out a handful of larger shells, made sure they were clean and empty, and dropped them in the tank.
Big hermit (we'll call him Rex) crammed into a suit he's grown out of.
Biggest of the selection offered. About twice the length, three times the thickness of what he's wearing.
Yesterday morning I noticed him again. Couldn't miss him; he was now carting around the biggest shell. And it didn't really fit, either. Too big, by far; he looked awkward, struggling over the rocks with this weight trailing behind.
Later on in the morning, I happened to glance into the tank. Rex was up beside the glass, looking over a smaller shell, in the same stripy pattern as his old outfit. (Not the same one, though; the first hermit is wearing it now. It fits her beautifully.)
Rex was rolling the striped shell over and over. Then, he poked his claws into the hole, both claws, as far in as he could reach. Rolled it again, checked out the inside again. Too late, I realized what he was doing and ran for the camera in the next room.
I missed it! He was inside that shell already. The big one lay discarded on the sand. Trouble was, the new shell, while a bit bigger, was still too small.
The time was 11:33:51. I was still watching, thirty seconds later, when he discovered another shell, of a different shape altogether. And here's what ensued. (The photos are blurry, because he chose an area where algae have populated the glass.)
"Look what I found!" 11:34:35
"Hmmm ... Nobody home." 11:34:42
"Empty all the way down." 11:34:47
"Looks wide enough." 11:34:55
"Let's see the back." 11:35:16
"I'll try it on." 11:35:35
Look carefully; you can see his red-brown hindquarters behind the striped shirt, wriggling out of the small shell. After this, there was a sudden convulsion. I took a photo that turned out just a blurry mass of brown, then ...
"Ah! Feels comfortable!" 11:36:15
"I'll take it." 11:36:31
And off he went, leaving behind the third outfit.
Total time elapsed for the entire "purchase": under two minutes. I checked back later in the evening. He is still wearing the elegant white suit.
"Suits me, doesn't it?".