Sick fish

[Jcj0033]

I bought a sailfin about 3 weeks ago he as doing fine until yesterday, I noticed that he had a few white streaks on his fins, i thought he might have just scratched himself but later in the nite it got worse he was breathing heavy stopped and eating, by this morning he was dead.
I need to know what happend to him ? What disease is this? will my other fish get infected ?
How can I keep this from happening ?
I have 1 goby 2 clownfish and 7 anthias in the tank now.
Tanks readings are
Nitrates 0
Phosphates 0
Ph 8
Salinity .26
Amonia 0
Nitrite 0
Here are some pics of my fish from yesterday to this morning

 

[Sniperhank]

It could have been anything from a scratch or an illness that he came in with. Looking at the final picture the injury looks more than a scratch and a pretty nasty cut that might have been from aggression, but there isn't anything in your current system that could have caused it. I'd assume that these injuries caused his death. Most likely from other tangs.

 

[melosu58]

White streaks on the fins are most likely ich IMO. It very well could be what hank said but it could also be the parasite

 

[Jcj0033]

Yeah, what's crazy is the first time I saw any damage to his fins was yesterday and it was a small scratch or small white streak you could barely see it, then later on that nite the white streaks had grown, by then I knew it was more than a scratch and by this morning he was dead and the white streak was now a huge white patch that is pictured in the picture. So I'm wondering it could be, and are my other fish in danger. I had the sailfin for 3 weeks, and my anthias for about a month.