A 75 gallon tank is the smallest tank that I would add a tang to. If you do decide to add another then choosing one that is of a different body type and color would probably be best. I would definitely not add any more than two. Also remember that tangs eat and poop a lot and need to be fed multiple times per day. Keep an eye on your bioload and wet skim and you should be fine. I think that when you are adding more rock and adjusting the territories it would be a good time to add the additional tang. Let us know how it turns out.
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