my buddy has alot of fish and i stop over and visit every month or so and hes had a 5 inch elong with a 6 inch rpb in a 45 gallon for about a year now and i asked him if they fight at all and he told me that when he first put them together the elong took a nice nip out of the rpb tail but since it grew back they havent touched each other and they sit side by side.. i do not encourage this mix but a year with no probs is pretty dang good..it would really be something if they made it 2 yrs.