Fish can be very delightful pets due to their bright and colorful body structure. But it can be very disappointing when your fish die for no apparent reason.
Fish are generally healthy and it is very rare for a tank to the subject of mass death without any reason. There is a variety of object that could reason tank die-offs, such as:

1. New Tank Syndrome
2. Rapid water changes
3. Water quality
4. Temperature Changes
5. Overfeeding

This is some common point that could reason tank die off. If you visit any Fishkeeping website such as https://fisharoma.com/ they also write the same which I mention here.