Man, Wolf -
I feel your pain. I had a similar thing happen when I fed some of my Cephs and smaller Neps with "Hikari Cichlid Gold" Fish food Pellets that comes in the RED pouch.
I'd been using the green pouched ones just fine (Hikari Cichlid Staple"), and they are a tad smaller. Two days after using the red ones, one each per pitcher, about 30 pitchers turned to a mushy black... aarrgghhhh!
I remember it this way now: Green means GO, Red means STOP.