I had 3 fishes die suddenly today. 2 dies yesterday. 1 die few day ago. They I suppose not attack by any fishes cos not really any body wound except when they die the fishes nibble on them. But they have one symptom which is the die with their month open wide. The fishes dead are fire gobies, royal gamma, goby, purple fire fishes. All are hardy fishes and have been eating well. My last fish die just mins ago and i see him till his last breath from healthy 30 mins ago. Can anyone help? What is wrong?