who's the culprit?

alright so i had 3 tangs in my tank, all introduced in quick sucession although not at the exact same time. they all lived peacefully for nearly 2 months, until about a week ago when i noticed my sailfin tang and mimic tang had some bite marks out of there fins. the other tang in my tank is a blue tang which doesnt seem to have any marks at all.
now the sailfin took to hiding all day long and only coming out for food after i noticed its fin damage and now i havent seen him for about 3-4 days(im pretty sure he's dead) the mimic tang doesnt seem phased, not hiding like the sailfin was but he still has chunks missing from fins. and i thought it could have been mimic and sailfin fighting but the chunks missing from the mimic seem to have gotten worse since the sailfins disappearance, so my only lead is the blue tang.
now who is the culprit? other than the blue tang and mimic tang i have 2 percula clowns, a mandarin goby, a firefish, cleaner shrimp and golden coral banded shrimp.
i dont see anything on that list that could have harassed my sailfin so much it died or picked on my mimic, unless its the blue tang?
am i correct in pointing the blame at the blue tang because i havent really seen any signs of it bullying anythin other than my firefish (who doesnt mind, just bolts for cover and comes right back out seconds later)

im quite annoyed actually because the sailfin was my favourite and i will be looking to replace him soon but dont want anythin else to meet the same fate.

 

well the blue tang can be it.they are known once they have been in an aquarium for awhile to become very aggresive.most of the time people add tangs at the same time so no one has a chance to make their territory.a day is a good enough time to make a territory.but are you sure you dont have a hitchiker in their somewhere?because if i remember correctly most tangs fight back.

 

I noticed a small crab the other day, looked to be a purple color. Although it was quite small so doubt it would be doing much damage, not enough to kill my sailfin.

 

yea definently not that.but i would at nite shut off all lights and wait about an hour and check it with a flashlight. a lil recon on the tank may show you who the culprit is.;D