i3k
Aquarium Advice FINatic
From the close ups of the black Oscar, it looks like he has some major scratches from those ornaments you have in the tank. They probably do get in a squabble at night and the black Oscar slams into those ornaments causing himself injury. And like others said, he probably has an infection now.
You might want to take those ornaments out and replace them with some soft fake plants.
You might want to take those ornaments out and replace them with some soft fake plants.