Within a few hours it was laying on the bottom of the tank. I picked it up and looked at it and noticed that its stomach seemed a little swollen and pale. I put it back in the tank and it swam sideways under some live rock. There has been no sign of it since.
So what happened? I didn't use the flash so I know I didn't stun it. So how can a fish that looked so healthy end up dead a few hours later? Could it have gotten stung by a coral? Could it have eaten a bristleworm? I have no idea. I'll bring a water sample to the pet shop today to make sure everything is okay. I have noticed that my Xenia isn't looking that great these days. It has stopped pulsing and looks kind of thin. Perhaps I need to start adding some iodine.
Update: I had my water tested later that day. 0 nitrites, 0 nitrates, pH 8.2, calcium 360ppm. Except for the calcium being a little low the water conditions are perfect. I have been dosing with Oceans Blend two part calcium. However, it is recommended that I dose before the lights turn on to avoid pH spikes. Well, I've been sleeping in turn my time off of work. Since the lights are on a timer they have been on by the time I wake up so the calcium hasn't been added for a few days. I also picked up some iodine at the pet store. I'm supposed to dose 3 cap fulls each week. We'll see if the Xenia starts to look better.