I have seven tangs in my reef tank, but keep in mind that my reef tank is 10-feet long.
Generally, tangs with the same body shape are most aggressive toward one another, but there are plenty of individual fish that are exceptions.
My achilles and powderblue are about the same size and really dislike each other. One rules one end of the tank, the other the other end. They chase each other back and forth at feeding time and about when the lights go out. If one or the other of these tangs is established, it is unlikely that you could add the other. Mine were in separate tanks in NYC, but when I moved the inhabitants of three reef tanks all the tangs were added together. The sohar, a very aggressive tang, tried to chase all the others out of the tank, but eventually gave up.
But, one never knows. I added a small purple tang to the tank which had an established larger purple. I expected trouble, but the juvenile just swam around with the larger one, and the yellow, who dislikes the larger purple, ignored the smaller one -- not a threat, I guess.
In general, add larger tangs last, if you cannot add all of them at once.
Terry
