Should I treat my other fishes ?

[Bossy]

Hi, My Klen's Butterfly wasn't eating for the last 1-2 days.. He was having some kinda shocks momentary.. but he was swimming and eating fine.. Anyways, yesterday he would hide and didnt't eat much .. Today he was taking deep breaths and died eventually ( had him for a month or more ). I couldn't spot any kind of disease at all.. Please look into the pictures and tell me if I need to treat the other fishes anyways. I have a 55 gallon aquarium, with a heater ( 78 F ), Aquac Realm Skimmer * Hang on *, 2 Marineland 350 Aquariums, a topfin powerhead.. around 40 pounds of live rock.. and 2.5 inches of sand bed.. live stock now I have is, 3 hermit crabs, 6 snails, 1 blue/green chromis and 1 Osc Clownfish. I've setup a 10 gallon QT tank.. Just in case I might need to use it. Please help me before I lose my other fishes.. i'm not sure what to do.. Maybe Butterfly was way too sensitive for a 55 gallon ? BTW, Ammonia 0, nitrates, nitrite are 0.. Though my ph is a lil low 8.0 .
Edit:
I just noticed my chromis breathing fast, thr's a brown ( cute like ) thingy besides his fins.. What could it be?
The pics are:
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Image - TinyPic - Free Image Hosting, Photo Sharing & Video Hosting