What is Killing All My Fish?

[kdragon64]

I've recently had a pretty big fish kill occur in my tank, losing half of my fish in less than two weeks.
Most of them looked healthy one day and were gone the next without any apparent damage.

However, this evening I came home and fed the fish and noticed this growth on my pajama cardinal's head just in front of his dorsal fin.
I've seen ich before, but that was always distributed on most of the fish's body, not concentrated in one spot like this. (see attached pic)

Does anyone have an idea of the proper diagnosis of this disease?

 

[Dmorty217]

Did all other fish have this growth that died? Did you QT the fish before putting them in the DT? Things that kill quickly in a tank are brook, flukes and velvet along with several others but those are the most common. Can you get a better picture of the cardinal?

 

[JustinM]

It almost looks bacterial and gram negative infections can kill quick as you described. A better picture will definitely help.

 

[Jdub968]

This looks bacterial or lymphocyst. I don't think it's the cause of a big die off Bacterial infections can kill fast but it's kinda rare for it to kill a lot of fish. Have you qt your fish corals cuc or added anything new to your tank recently? Also any heavy breathing scratching flashing