stable tank with dying fish

One day he was fine and last night he looked like this . He’s not eating. Was he attacked by a hermit crab? Is it a parasite or bacteria? I also have a clown who looks fine and a tangaroa goby who looks fine. What happened to my poor fire fish?!? He is 1.5 years old. Is there a chance he could...

Hey fellow Reefers.. although im not a reefer yet, just an aspiring one, I have a freshwater aquarium which I am using to learn ALL (!!) I can about to graduate to a reef so I don't mess up. But now seems that im messing up! Now in the last weeks one by one my fish are dying off.. why?? Fluval...