apple, pls go n do some research or read the articles about the betta n how to keep a betta before you go n get one. I believe the water parameter went wrong at yr place. PH, Chlorine or Copper is the main issue.

And most of my fishes was died due to my house area implemented with few giant size of copper pipes. And the water contained the copper which not really harm to human being. But it is harm to fishes.