I am assuming they are both Mbunas? Mbunas can be very aggressive/territorial, depending on the size of your tank they should be in a group to kind of spread the aggression a bit rather than just picking on one. Example, I have kept 12 to 15 mbuna in a 55 gallon for years, yes there is always one bully but with careful selection they can be fine in a large group. I first started with 5 and it didn't get any better till I had about 12. Then again, what works for me may not work for you. What size tank do you have and how many fish?