Who's the bullie?

[Willito]

I would be most interested to learn how you, with your experience, was able to predict and calculate the instinctive territorial aggression levels between the two acanthurus species.

Nothing is scientific nor by the book here, but has more to do with observation and using common sense. To introduce a new fish into an established tank is a big risk, so by lowering that risk the chances for survival will increase. That new fish must be healthy and in tip top condition before entering the Main tank. What I do is quarantine the fish until it is accepting various foods to accumulate a substantial body fat. This fat can be utilized during the first few days while it is getting used to the surrounding and may not be eating. Or in this case, being chased around.

Once the fish is fit enough, I introduce it into the main tank after the sun sets. This is important because it allows the new fish atleast 12 hr to acclimate to the new environment without stresses put on by other fish. At the same time, fish tend to be a little more sluggish to start any kind of war. Re-arranging the aquascape is also ideal for this condition but I don't have that luxury.

When the Sun comes up, I make sure that I am around for atleast two hours to watch. The initial confrontation with the new and old fish is inevitible. Let mother nature takes it course. If you're lucky and have prepared it well, it may not be dominated by the established tankmates.
My situation was of the contrary. The PB was eating and was healthy, but the one tankmate was fierce. As expected, the Sohal won the war and took dominance over the PB. Tore it's fins and confined it into a corner.

The next 24 hours is critical in determining whether or not the PB will be compatible in this environment for short term. Observing the behavior of the Sohal and PB every hour while the light is on is critial. If the Sohal was reluctant on chasing/beating on the PB, then it's obvious that it won't survive and one has to be removed. In my case, the PB was confined in the corner for a few hours without further harrassment by the Sohal. As time went by, the breathing of the PB returned back to normal and it started inching out of the corner. By night fall it started grazing on nearby rocks. At this time, the Sohal would occassionally swim by and flared it fins, but no contact would occure. This is a good sign.

On the following day, I placed two pieces of Nori into the tank. One at one end for the big guys and one in the corner where the PB was hanging.
Everyone had there fills without incident. As the day progress, the PB ventured a bit more into the open under the watchful eyes of the Sohal. The fin damages are slowly healing and body weight is slowly back to where it was when it first enter the tank.

Today the fins are in better shape, the stomach is full, the colors are brilliant, and the swim is much easier than anytime before. The PB is now eating from the same Nori clip as the rest and is scavanging on one half side of the tank. Hopefully the entire tank by week's end. Long term survival is looking a little more promissing now.

No hard feelings Chin, I guess I should have described a better picture throughout. Peace!