Major dilemma w/ multiple dying fish!

[tah532]

Ok, here is the situation. I recently placed a Yellowtail Blenny, Flasher Wrasse, and Orchid Dottyback alone in my 75 gallon tank. They were in together for 2 weeks and there was nothing but harmony. I then put in a Cream Angelfish, which didnt look to be harassing or bothering the 3 other fish at all. The very next day the Flasher Wrasse is dead and the Yellowtail Blenny is acting very aggressive and constantly going after the Angel, seeking him out and actually swimming after him. So I remove the Blenny and take him back to the store and trade him back for a Juv. Mimic Tang who is nothing but healthy and energetic the whole day after adding him. The Angel again did not seem bothered by the Mimic and left him alone. Now this evening my wife, over the phone, tells me she has not seen the Tang all day and he is no where to be found. WHAT IS GOING ON? I wonder if its the Cream Angel but he does not act aggressive and just swims along the rocks all day and never visually went after anyone. Does he turn into a monster at night? Or could it be the Orchid Dotty killing everyone? The Orchid has been in the tank for the last 2 years and never bothered past residents. Any ideas would be greatly appreciated.