Yeah, Neon tetra disease is UNcurable. And suppose to be catchable. You need to separate the suspected fish immidiately to the other small tank or container. Hospital tank. Like only air-stone and maybe some plant(some people use the plastic one) and maybe sponge filter. And start treating with chemical or salt bath. Although like I mentioned earlier. There are no known cure for this desease. If your diagnose is right, you cannot cure them, unfortunately. But if the fish is sick from the other disease, salt bath may help since it is effective cure for many disease like ick. So try salt bath and slowly increase the salt content by day by day. Also need to change the water frequently.
But if you see the fish lose more color and become white and start to lose the weight and especially tail become dwarfed and have tough time swimming or develope some breathing difficulty, you should put it to sleep. Some recommend to drop the fish to boiling water, some recommend to give blow in the head, or cut the behind the head with sharpe knife or add enough alchool in the water. Sigh.
Not a fun subject to talk but it better off to put to sleep when they develope swimming or breathing difficulty. Especially, neon tetra diesease is un-curable.
I hope you are wrong with your diagnose and it is some curable diesease but separate the fish IMMIDIATELY. So rest of the fish will not catch the diesease.
