What is suppose to do is replace minerals lost to the RO/DI process. It is for none planted tanks, where as equilibrium is for planted. What the difference is I do not know as I don't know American cichlid salts.
From what I understand Replenish is for people who use RO water for their tanks. RO removes some essentials nutrients which Replenish claims to replace.
I don't use it so can't comment on its usefulness.