a4b972ab_m_and_Freshwater_Fish_Problems__Problem
[Symptoms] One fish dies showing no symptoms. All other fish remain healthy [Solution] It is often difficult to pinpoint the cause of a problem when only one fish is affected, particularly if it showed no symptoms before death. The cause can be any of a number of factors including an internal infection, worms, bacterial infection, malnutrition, or old age. In most cases there is nothing special that should be done. Keep a close eye on the other fish and if they begin to show any symptoms of disease do a partial water change and treat accordingly. Most of the time it will not be contagious. [Problem] 
One fish dies in a healthy tank
Aquarium and Freshwater Fish Problems
Problem