Sudden red discoloration and death is usually ammonia shock. How do you know your water's fine? What levels were found on the test results? Was there anything done to the tank in the last couple of days? Were any new chemicals used in the house? (Easy-Off on a warm oven can kill whole tanks off from several rooms away). Was anything accidentally dropped into the tank (like a cleaning sponge/rag)? Do you have any pics of the red areas? Was the knife feeding up to its death? Any other details you can furnish would help in determining the cause.