My Two Male Cats will not stop fighting! Help please!?

About 2 weeks ago, we had to put down our 18 yr old female cat. Ever sense then, my other 2 male cats who are about 4 yrs old have been fighting. They are both fixed but ever sense we put her down, they keep attacking each other. They also act very differently as well. This never happened before. Not sure what to do. Wasn't sure if getting another female cat would maybe help and calm things down or...?? . Any information would be helpful!!