Fish die without obvious warning in an established tank.
Short answer
Work through the physical and chemical causes before the biological ones. Most unexplained deaths in established tanks trace to something that entered the water, a temperature failure, or a slow decline in water quality that was never tested for.
Ordered by urgency, then by how commonly each one turns out to be the answer. The discriminator is the observation that separates this cause from the others — use it before acting.
How to tell: Temperature well outside the intended range. A stuck-on heater cooks a tank; a failed one chills it.
How to tell: Sudden deaths across species with clean water tests. Aerosol sprays, insecticides, paint fumes, cleaning products, or hands with soap residue.
How to tell: Deaths overnight or in hot weather, often with survivors at the surface in the morning.
How to tell: Infrequent water changes over months. KH near zero, pH low, nitrate high. Deaths often begin after a belated large water change swings everything back.
How to tell: Deaths began one to three weeks after new fish were added, and spread through the tank.
AquariumHQ describes what is commonly reported and what husbandry correction is usually indicated. It does not diagnose and is not a substitute for an aquatic veterinarian. Many treatments are given to healthy fish suffering from a water-quality problem, which makes the outcome worse rather than better.