fishes dying

Question
hello there i have a 175gal. tank i started with 3 damsels and bunch of snails and hermits tank seemed to be cycled after bout 1 and half months, soon after i purchased 2 mid sizes powder blues and they were doing fine together as i watch for next 2 weeks soon after that i got a kole tang and yellow tang and they seemed fine for the next 2weeks or so. everything was fine for bout 2-3weeks then 1 of my powder blue died[colors of the fish seemed to be fading/cracking] then next day my other powder died and the kole tang,yellow tang my damsels seem fine and my clown seems to start fading/cracking in color i test my water weekly and the ammonia nitrite ph seems all fine and i was feeding them brine shrimp and sometimes mysis[freshwater] and algae here and there. ihave a uv sterilizer and protein skimmer. im lost in whats happening to my tank is it the food? and i do have some corals and they seed to be doing fine.

Answer
Powder blues are very delicate and do not travel well. Koles are fairly hardy but still delicate. the yellows are very hardy and should be okay. Clowns are also [depending on the species] delicate, but it does sound like a virus or bacterial infection. Check your nitrate levels and check the levels of the water the fish come in to see if the water in your system is a close match. Also never add the water from the store where you buy your fish. It could be contaminated because of the coming and going of so many fish. They usually treat their water with copper to kill bacteria, etc. You also should only have one addition at a time and cycle that new one through a separate smaller tank to make sure it is eating and not diseased. Also when adding fish are you acclimating them slowly? Get back to me, Gerry