[Myka]

How long has the tank been setup? How long was it setup before you bought it? How big are the current fish (the tangs, and trigger)? What skimmer do you have?

The Sailfin Tang will outgrow your 120g, so I would think of rehoming him before adding any more fish. They grow fast too. Is your tank a 4' or 6' 120? If it is a 6' you could consider adding another smaller tang, maybe some reef-safe wrasses (some are big some are small). I wouldn't risk an Angel (coral pickers). If your tank is a 4', consider adding smaller fish (under 5").