Man this is crazy. Just a few days ago a buddy of mine told me he bought a copperband and when I told him dude why didn't you ask me take him back he said it just disappeared. I went to his house a few days later and zero signs of him.
I can understand a smaller fish. I had a bartlett anthia turn male when there was already a male in the tank. They fought and fought till eventually one gave up. He wedged himself between my rock work where I could see his tail, but not enough to be able to pull him out. Few days later disappeared. Then about a week later I saw his skeleton floating around. I grabbed my net to pull him out and just as i was removing my screen top my wp60 sucked him in and made the entire skeleton vanish.
Honestly though, 2 decent sized fish, no sand, clear views. .. i think it's time to break down and inspect every crevice of every rock. Unless you have a cat that is getting a late night treat out of your display there has to be something in there that shouldn't be.
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