mysterious fish death

Lost my first fish yesterday, absolutely gutted as my little tailspot was a real character and the tank feels much quieter without him. Looking for any possible advice on why he might’ve died? The tank mates were a clown and a hectors goby, all of which he would regularly swim with and get on...

Would like people's thoughts on this. I have been keeping a mixed reef tank for years now, and have always fed my fish with mainly frozen foods! Last night I fed them with frozen krill, and to my surprise a white feather was introduced into my tank via the frozen krill. The very next day my...