Help... Sick Male Fighter – White Scales Under Head

My Fighter has developed white/grey/silver scales under his head. He's been hiding all day, have transfered to a jar. Water was borderline in need of change, and have added 50% treated tap water to the jar .He's staying at surface and floating listlessly. Doesn't look like he's going to pull through, any help appreciated, pic attached.

Not sure what is wrong, maybe looks like a fungal infection, my biggest concern is the jar, does it have filtration? Ammonia levels can shoot through the roof quickly which wont help at all, and could easily become the cause of death.

There's not a lot of info here to determine a cause, what was he living in when he got sick? When you say the water was in need of a change was this due to ammonia/nitrite/nitrate? As Laurac94 said ammonia can build rapidly in such a small container, until you can determine a cause if the illness clean, warm water is crucial.