aggressive oscar

Question
Hi james,
I have 2 oscars they use to be the same size.  but one got sick and is now 1/2 the size of the big guy who is 3 inches long.  ever since he got smaller  big guy is picking on him; it got so bad that the pet store was nice enough to take him back.  I was told to get one the same size.  He was picking on him to; I dont know if it is because i did the exhange on the same day or what.
So I brought him back.  the pet store told me because he was so aggresive i should get a 3.5 or 4 inch oscar to pull some of to agression away from him.  55 gallon tank 3inch pleco and 3 inche oscar.   what do you thing

Answer
When you put another fish in the same tank with an oscar that has been there longer your oscor will show dominence in the tank.Especially if the fish is smaller.Need to get one as big or bigger.Then maybe the bigger fish will show him he's not gonna be pushed around.An oscar is one of the most aggressive fish there is.You need to find one  just as aggressive.If you don't they will not survive.
And they reccommen 55 gallon tank for each oscar.

Hope this helps.