The gravel you are using isn't sutable for Corydoras and is most likely the cause of the trouble.
The stone you have is a course volcanic rock which is very sharp and will injure the barbels, fins and underside of the fish.
You should change the rock for a smooth rounded sand material and the fish will be much happier.
Best Regards,
Bob
Thank you for your comment Mr. Bob. For this matter I've just moved out all Corydoras to the other tank that using ADA soil instead. I didn't realized that surface material will causing injure to fishes. So I'm planing to remove volcanic rock and re-arrange this tank for next month.
For most of my tanks I just use glass substrate because it is easier to clean the tanks, but several species do better with a thin ( less than 1 cm ) layer of fine river sand. River sand is smoother than sand made from crushed gravel, because it doesn't have the sharp edges anymore.