Sick oil cat- need FAST answer or am finding the clove oil

I recently purchased, among others, three oil catfish. They were doing just fine for a while, but now one looks really sick. It was flapping around at the top like its tummy was full of air. I thought that was the trouble at first- I've had cories do that, and they're usually fine after they spit it out- but now he's much worse. His colors are really faded out, he's stuck at the top, and he doesn't move unless I bother him. His fins are flared and stuck in the "I'm scared so I'll be spiny" pose, as well as his mouth being open really wide. He's breathing, but it looks kind of shaky and not right. Any clue what's wrong? I think he's really freaked out, and I'm not sure if he's hurting... If I don't get a good answer, I'm gonna have to put him to sleep.