Toxic Water
I have had the same problem with chromis myself. I purchased seven and now only three remain. It seems the shy chromis become overwhelmed by the more aggressive ones and they kick the bucket.
The problem you are having in your tank sounds like a toxin may have entered the water. Have you used any airbourn chemicals lately or intoduced any cleaning sponges that may not be aquarium safe or have hidden chemicals in them? This has happed to me in the past with a "3M" sponge that didn't have any warnings about it not being aquarium safe. It wiped out my whole tank in hours.
You should do a large water change and if you have a separate tank that may have not been affected, transfer your fish ASAP. Do you have sufficient water movement and protein skimming? If you have any live rock, it will absorb the toxin, as well as your live sand.
Hope this helps out. Good luck.
Stan
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