Gold Laser Has Turned White...

[Harlequins]

One of my Gold laser corys has turned completely white, the same thing happened to another gold a few months ago and it unfortunately died

Any ideas what may be causing this?? - 55 gals has been running 3+ years with more or less the same stocking,mainly cories...so well cycled and regular weekly w/c etc , ammonia/nitrite 0, nitrate 40(tap 10)

 

[snazy]

What's your Ph normally? Any temperature changes?
Poor thing. Have you got another cycled tank that you are not afraid to infect with whatever it is just to make sure it is not something in the water causing it?