hey there.. how big is your tank.. you definitely cant buy anythin smaller than a 12" aro and try introducing with the existing aro.. aros are usually kept in odd numbers 1, 3, 5.. and then there are people with two aros also and everything seem to be fine. i have 3 aros in the same tank.. 2 are around 18" and 1 must be 16"+.. they had their individual tanks till they were about 12" to 14".. i did try moving them with my other large aro but she whacked them within 30 minutes of the move.. so exercise caution.. best thing is if you have a real large tank.. you could divide it and put em in each half.. try introducing slowly.. any sign of aggression get the divider back on.. cheers..