adding adult fish

I have 55 gallon 4 foot tank. my oscar js about 8" and I added a vieja which is about 10" to the tank but the oscar went at him bad for like a minute and took some scales off the vieja. the vieja is a female. I have now put vieja in a different tank. what is the best way I could add her to...

I have had this Uaru for 5 years got him as an adult he was a rescue and I never wanted to keep him this long. He ended up killing my severum stressed him out to death I should have gotten rid of him then. This fish once was nice and ok with tank mates now he is a jerk even to the silver...