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I saw a 2.5inches damsel at the LFS. But was told it can be aggressive should I get for my tank. Current tank mates are percula clowns, yellow tang, and algae goby
Damsels are good beginner type fish as they are very hardy but three versions I have had in the past became very aggressive and territorial even in my 75. The Domino, Yellow Tail and Convict (black and white striped) were the three I had at any one point in time. I had to finally turn them back into the LFS as they were really beating up the other more timid fish.
In a 90 you have the room but I wouldn't have more than one or two at most. That gives them 45 gallons of "space" apiece