IMO betta's arent well cared for before they get to their final home, so anything could be wrong with them , and its unlikely that it is your fault. puffers are one of the more difficult to keep period. i might suggest if you are considering getting out of the hobby, but still enjoy fish, to try an easier to keep species. one thing to know, is you are not alone, we have all had fish die on us, and sometimes its our fault. it is part of the learning process.