Hey Andre. I've not owned Zebra Cichlids, so I'd recommend posting a question in the Cichlid area. The one that you had in the tank first is viewing the tank as his territory and the new fish as an intruder. Sometimes taking the older fish out of the tank, rearranging the decor and acclimating the newer fish in. After the new fish has been in a hour or so,
acclimate the older fish into the tank but be prepared to remove one if they show aggression. This is just general advice and might not work with zebra cichlids, so please post in that area to get the opinions of zebra cichlid owners.