Well do you guys know any sites that give some valuable information on some of the fish sicknesses and illnesses. And some of the most chronic ones that surely lead to death?

Also I have a gold fish which was doing fine on Sunday night and on Monday Morning. But, when I came back from school Monday Afternoon the fish had like whitish ripped up scales over it in different sides, and its tail fin was like rotting and seemed shredded up at the end. It was like it was losing its skin, like you know when a human gets sunburned you start losing your skin. Well yea like that. It still is like this, but its gotten better after we've changed the filter, and the water was a bit too warm. But, I dont know.. Anybody can help me with this? Oh it was swimming near the bottom too, but right now it isnt. I have 3 goldfish in the tank.