fish disease..

[Tram]

My parents' goldfish had some kind of disease that made a black spot and several blackish markings appear on its fins and tail. Before the day that my parents bought some kind of goldfish medication.. his whole body was deteriorating. Before he died this morning, more black and white markings were all over his body. I was wondering.. what kind of disease is it.. and how would I be able to treat the tank in full so my fish won't get infected. I'm starting to notice that my parents' other goldfish is infected as well.

I have a snow discus, 2 aussie arrows, 2 needle nose, an angel, a chinese algae eater, and a black ghost.

My parents have 3 kois, and one remaining goldfish now.

Thanks.