Mysterious disease

[KrazyKuch]

I must have some strange disease in my fish tank but I don't know what it is!!!

The last week I have lost 3 fish, to unkown reasones. Today being the most recent, I woke up this morning and all my fish seemed fine got home from work and my Lemon Peel angle is dead lieing on the sand....She had no wounds no visible disease just dead.....Did water tests last night and everything was fine except for nitrate which was at 20ppm (also have a little cyano), so I did a 20g water change.and I just checked the water again and same thing everthing fine except for nitrate which was at 10ppm.

What could be killing my fish????