Mine spent 3 weeks in QT
. Ate frozen foods a little. All of his eating was very brief -a couple of mouthfuls and done. Thinking he was OK, he went to the main. Moved in and out of the rocks constantly, so I'm thinking something was wrong with yours when you said he stayed in the corner for long periods. They constantly move, searching the rocks for pods
to eat. Mine did that, a got a fat belly. Funny though, I still never saw him eat - just going thru the motions and getting a fat belly. 1 week later, dead.
How old is your tank? Not QTing either?
Best of luck to ya!