How's the fishy? Are you doing regular water changes?
Just a suggestion, but one of the things that was said here that it was probably not a good idea to get a fish that didn't look well to begin with...just keep that in mind next time your shopping for fishy...
try to get a good look, I know not always easy, and even the healthy looking ones can die once you get them home...but try to watch for anything that might indicate a problem not swimming much, physical signs like missing parts of fins, eyes not clear, mishapen body...if you know what I mean!
Test your water...water changes....don't feed too much...and know that sometimes, no mattter what we do...sometimes our fishy will die and sometimes we just don't know why
Good luck!