Chromis's dropping like flies

[melellaquarist]

I've had green chromis for a while, but during this period three of the six I originally purchased have died. The first one we found belly up, the second we found dead at the bottom of the sump, and the third just vanished. Now one of the chromis' tail is being covered by scales. I can't tell if the scales are growing over the tail or if it is a fungus. Does anyone know what this is or why my chromis are biting the dust.