That's definitely what i was shooting for, because there's comfort established in the environment already. One added fish with aggression could turn the table's fast. I have to say if it's a warm feeling you have about it, no one's holding you back. Believe me it's more difficult to loose a fish you had time raising, rather then buying a fish you don't know what problems will exist when adding it to you're tank.If you have harmony in the 120 gallon I would pass on the JD because adding it may cause problems. Also if you had to take JD back the store will not give back what you payed for it.