Last time mine quit feeding it was a bacterial infection. Do you see any change in behavior just before death, swimming sides, gasping, sores? And the infection may not be apparent at onset, check the deceased ones very close for any signs of an infection. If you find any infection, quarantine them and treat with Tetracycline, available at most farm supplies. It's listed for birds, but works equally well on fish too. I don't remember the dosage off the top of my head, but I can get it if you need it. I lost about 8 fish over a week before I was able to diagnose mine.