well, the brown part is sorta confusing, since there's not much that would turn a fish brown.
possible problems:
you acclimated the way i do, but not enough time. i generally do it every 15 mins for 1.5 hrs for a hearty fish (clowns, etc) and up to 2.5 hrs for a not hardy fish (tang) but also in a much larger bowl for the larger fish.
basically you're trying to make sure the water in the bowl is as close to the water in the tank as possible.
i'm not sure on the hawk, but if it died so quick i'd assume acclimation problems.
quick deaths like this are generally bad acclimation technique, ammonia issues, or even collection issues. did you watch either fish in the store for a while or just get them? were the fish eating at the store?
also, your maroon clown probably knocked the crap out of the fish without you knowing it.
i'd also suspect ammonia issues. you have a nice big bioload already in there with the maroon and the lion, and subpar amount of live rock. the existing fish may be ok with it, but adding a big pooper like a kole tang in there, plus having a fish die in there, i'd bet you had an ammonia spike you couldn't test for in time.
no tang should be in a 55 gallon tank, even a kole.
all these variables could have been avoided if you quarantined your new additions. no aggression, controllable ammonia addition, able to watch and observe any odd behaviors, etc. plus who knows if you added ich or some other disease into your existing tank--you put your lion, clown, and chromis at risk especially since tangs are notorious for them.
