What is wrong with my neon tetras?

started up a neon tetra only tank.all was going well.water quality excellent,the 25 neons i bought from local pet store seemed ok. everything was fine.until i decided to buy 20 neons online as it was cheaper.they arrived ok put them in my tank but in the last few weeks i have started losing 1 neon every other day or so! have noticed some have lost their colouring and have a dusty sort of look to them. i have treated them for whitespot but it seems not to have made a difference.please does anyone have any ideas whats gone wrong? :-(