Well, the little guy I've been calling Pale-y was put to rest today.

He'd spent most of the day by himself in the bottom corner of the tank. When he did come out to eat, he seemed to be struggling to swim so it looks like he had NTD for sure.
Do I need to do anything to the tank to help prevent the others from getting it? All look to be in great shape and all water parameters are excellent.
The vodka method recommended by Wilder seemed relatively quick and painless so if any of you ever need to euthanize a fish, it didn't seem like too bad of a way to go - fix a shot for yourself first.
In memory of my little friend --
Limerick for Pale-y
There once was a fish named Pale-y
Who dreamed of becoming a whale-y
His red stripe grew smaller
But he grew no taller
Alas, now he's dead as a door nail-y.
Rest in peace my little friend.