The clown knife was also a suspicion but it has some marks on it as well. Our Oscar is very *****y and always has been so I think it's him picking on them. He's an ******** sometimes.
Get him out until he gets bigger. Once he's larger, he will be king of the tank... When you put him in the other tank, add medicine to help him heal. If you leave him in the tank, he's gonna be more stressed there than in a separate tank