Anytime you add a new cichlid (or cichlid-like fish) to a tank where there is an already established cichlid (one that feels the entire tank is its territory) you should expect the old resident to attack, and maybe try to kill the new guy.
In nature, most average sized male cichlids will defend a territory of about 250 gallons, and as soon as the interloper moves out of the size area, the attacker breaks off the attack. A 75 leaves no such leeway, and often the attacks continue ad nauseam.
This is why most experienced cichlid keepers don't add cichlids piece meal, but instead, get a group of juvies, add the all at once, and the juvies will carve up territories within the limited space as they grow sometimes establishing a hierarchy within a smaller space out of nessecity.
Redocorating/rearrangeing the tank just before adding a new cichlid sometimes helps, confusing the old resident, and allowing a level playing field for the new guy