something destroyed my whole tank, please help

[hqgc]

over the last 2 weeks something has wiped out my fowlr tank. I have lost all my fish with the exception of my bluelined trigger who is not looking very well. The blueline is currently sticking its mouth out of the water, almost like its gasping for air.
Symptoms include
-cloudy eyes
-extreme discolouring and patchy "paleness"
-fin rot
I have tried melafix to no avail, and am now at a loss. All my parameters are good.
I need to make this better but have no idea where to begin.
Any help would be greatly appreciated.
thanks