Seems weird. I had 3 chromis and they seemed fine at first then one wouldn't come out and eat and a few days later the 2nd one did the same. Now (3 weeks later) I only have one left (the one that was the bravest and ate the most the whole time) and he started to hide also, until I put a clownfish in there..instant buddies!
My experience was different than yours as my fish just suddenly disappeared...so I don't know why your chromis is dying
Btw did the diatoms go away and what corals do you have?