Cichild deaths ... Is this normal ?

Hi,

I have a 150gal with 32 mixed-lake africans.  Over the course of the
last year and 1/2, I had 6 of them die on me out of the blue.  At any
point when any one of the 4 dies, all the others are still healthy. 
The symptoms are basically spitting out food, fins are frayed &
tattered, hard breathing.  I can never catch them to put them into an
isolation tank so they eventually dies.  Is this just a normal thing
that some fishes dies over the course of keeping the tank, or do I
have some problem lurking somewhere ?  Thanks.