First of all, I love Tangs. I have:
2 Paracanthurus hepatus (Blue Hippos) 4-5"
4 Zebrasoma flavescens (Yellows) 4"
1 Zebrasoma desjardinii (Desjardinii tang) 7"
1 Zebrasoma xanthurum (Purple) 3-4"
1 Acanthurus sohal (Sohal, black-spot) 3"
1 Zebrasoma veliferum (Sailfin) 1.5"
I started acquiring them with an eye toward a new much larger tank I was building and my goal was to introduce them all together so as to eleminate any pre-existing territoriality. After housing them in a variety of tanks I moved most of them into my new 450. There were some routine squabbles as the pecking order was established but all went really well.
Any fights, I reasoned, would probably break out among similar body types, Zebrasoma vs. Zebrasoma, Paracanthurus vs. Acanthurus.
I added the second Hippo a month later and all went quietly. It was a non-event.
The final Yellow added was brutalized by the other Yellows and the Purple until a truce was called, but with the truce among Yellows, they tried to shift their focus to the Desjardinii.
Bad move! He's twice their size! That fight fizzled quickly.
I've read that as they grow, Tangs will frequently retest their place in the heirarchy. I've seen the peaceful little Sailfin take on the Desjardinni, immediately lose, and then normalcy would return.
My LFS guy is also a Tang lover and I asked why the Sohal (supposedly one of the two most aggressive I had, the other being the Purple) had never had any conflicts. He smiled and told me to wait. As the Sohal continues to grow (his is over 10") he told me to expect that he'd eventually become the Alpha fish in the tank.
Never a dull moment!
