Nick , If you are dead set on keeping these fish together , consider getting another 30 gal [ long tanks are best IMO ] and keep them end to end next to each other .Or buy a 55 , 60 ,or a 70 gallon tank . How would you feel If everywhere you went you had to run for your life or fear of being beaten , only you have nowhere to run . I hate to put it that way . But that is what you may be subjecting your fish to . IMO , It's not a question of over stocking as much as over crowding , I beleave there is a difference[ guilty of both in the past ]. 2 different species of fish the same size may not require the same amount of room to live in comfort , and add to that breeding pairs of fish ." THE CICHLID ADDICT " made a good point , you should try your best to make your fish feel safe and secure . not just keep them alive . By keeping the tanks near each other the fish may see each other thru the glass and act aggressivly as to defend it's terratory with out any physical confrontations .If it looks like it may still cause a problem , slide a piece of paper in between .