Corys are schooling fish and like to be in bigger groups. I'd suggest getting 1 or 2 more corys. Maybe that will perk the smaller one up.
Agreed. Ideally, cories should be kept in groups of 10 of the same species
A couple more should probably help your little guy though :)
What type of cories do you have?
I have trilineatus, loxozonus and pandas :)
Several are wild caught in South America... not bred in captivity. So right off the bat, they have a LOOOOONG trip to get to your tank.
Also in the wild, they school in the hundreds. So, going from hundreds of your buddies in the wild, to only 3 buddies in a 10g tank can take a little "adjusting."
Another thing to watch for is cories only breathe air when the dissolved oxygen in the water is too low. If you see them breathing at the surface frequently, think about adding an air stone.
Get a couple more. And as long as it's eating and breathing through its gills, it should adjust to your tank. GOOD LUCK! Cories are super fun and have GREAT personalities!!