as kien mentioned they are a tricky fish to keep alive long term. If you do end up getting one to stay healthy in your tank i think you will have a very hard time adding a powder blue and orange shoulder once the clown is established. Acanthurus tangs generally do not get along very well. I don't think your tank is big enough for 3 acanthurus tangs or 7 tangs in total. I have 6 tangs in my 280 and i already foresee them outgrowing the tank down the road. I hate to sound like the tp, but imo i don't think your tang list will work.
+1
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One more fish should be ok?, right!!! - Laurie