Hi Planoi,
I made some major mistakes with tangs in my 75g tank in the first year that I started in this hobby and would like to share my experience with you and the others on the board here.
After the tank was a few months old I added a Scopas tang (Zebrasoma scopas). He seemed to be a very sturdy fish and really active.
Well after a period of time I felt I had to have another tang so I added a Naso (Naso lituratus) that was about the same size as the Scopas. After about 2 minutes in the tank the Scopas started to constantly harass him and about 3-5 days later the Naso was dead. I feel that the shape of the Naso's mouth was what caused the Scopas to attack it and whenever the Naso would try to graze at the algae the Scopas would attack.
Then I tried a Yellow tang (Zebrasoma flavescens) which was a little larger than the Scopas that only lasted for about two days. The reason for this is that the Yellow and the Scopas looked identical except for the coloration.
Next came the Achilles tang (Acanthurus achilles), which was a bad choice in the first place because they are so hard to keep anyway, which made it for about a week. The Achilles had a different body style but was the same color as the Scopas except for the orange tail and fin area.
You would think that I would have stopped by now right? Wrong.
Next was the really nice Kole tang (Ctenochaetus strigosus) which lasted about 4 days. Again simular coloration and mouth type.
Then there was my wife's 6 month old Sailfin (Zebrasoma veliferum) tang that we moved from a smaller tank into the 75 and he lasted about two weeks. Here again simular body type.
Then came the large Naso (Naso lituratus) which also was gone in about a week. I thought that by having a fish much larger than the Scopas would solve this problem but it did not and the end result was the same.
Finally after killing all of these really nice fish, and feeling like a real jerk, I added a small Regal tang (Paracanthurus hepatus) to the tank which has now been living peacefully with the Scopas for over a year now which I really think is just luck. The only reason that I think the Reagal and the Scopas are living together is because the body types are so different and the coloration of the Regal is also different than any of the Zebrasoma species. This is not to say that having these two fish together in such a small tank is a good combination either. There is a always a high percentage risk that once the Regal tang reaches adult size there could be a serious turf war between it and the Scopas which will end in the death of either one of them or both.
I am also somewhat sure that many of these problems came about from the small size of my 75g tank and the Scopas defining the entire tank as his own territory. With the Zebrasoma species defining large areas in the ocean as their own territory keeping them in our small glass boxes stacks the cards against us from the start.
Not only did I lose all of those other tangs I also lost other fish because of the constant stress of the tangs fighting all the time.
I hope that things work out better for you than they have for me in the past but try to keep a close eye on your other fish for stress related illness and make sure that they are not going to become victims of the two tangs fighting.
HTH
Doug
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Doug's Reef and Fish Page