Sick fish

[Fire.is.fun]

Last time I was at Blue world aquatic I noticed a lot of sick fish in the display, but like a fool I still bought some Corel frags and a feather duster. Two weeks later and now all of my fish are dying from a parasite or something. I thought it was ich but it must be something different because a healthy fish one day is dead the next morning. Has anyone else experienced this and has any idea?