Sick Neon Tetra

[tahl]

Hello, today whilst observing my fish tank I noticed a white discolouration on the tail of my neon tetra's. Worried I looked up on the internet to see what the cause is and I came across Neon Tetra Disease but I'm not sure and seeing as the only advice I've found on how to treat it is quarantine or euthanize the fish I want to be 100% sure that it is indeed NTD. My major concern is the tetra's tank mates as I do not want the disease to spread. I have a 64L tropical aquarium with 1 male siamese fighting fish, 4 neon tetra, 5 harlequins, 4 silvertip tetras, 2 cory cat fish and 2 borneo sucker loaches
Here are my water parameters:
pH - 7.2
ammonia - 0 ppm
nitrite - 0 ppm
nitrate - 0 ppm
tank temperature - 26C (78.8F)

 

[Sickbound]

Not really related to the neons but you should have 6+ corydoras for them to be happy. I suggest 10+ but 6-8 is fine.