I came home from work today to find my smaller oscar with a huge tear in his tail fin and top fin. He didn't have them when I went to work in the morning so I am confused if this could happen over the course of 12 hours. The tear in his tail fin is a good inch and a half long and clean down the middle.

On a side note my older bigger oscar has been aggressive lately. both of them are coming off successful hole in the head treatments as well so I don't know if that has anything to do with it. I checked my chemicals and everything was ok so I can't pin point what is going on.

It may be aggression, and it may be an Oscar being clumsy! As long as your water is fine it will heal quickly!...

If it keeps occurring or other marks show up, then you might need to think about separation.