After 40 years as professional and 50 years of keeping tropicals, the following are the biggest killers by far.
1)Adding chemicals to your aquarium to 'cure' a disease, this is mainly because of the blind faith placed in companies who make medications for fish keepers. Most meds should be used in quarantine tanks or as baths only. Common sense tells you if its designed to kill microbes then it isnt going to help your filter.
2) Doing something.........More fish die because people do something than have ever died because the keeper did nothing and watched. Reacting to a few white spots is a recipe for things going wrong.
And the biggest killer of all is poor water quality, just about most problems can be traced back to water quality.