Neon Tetra Disease
Neon tetra disease is a set of bacterial infections caused by a parasite that is normally seen infecting neon tetras, but they can infect a wide variety of different tetra species fish. Sadly, most if not all of the disease's signs are shown only a few days before death. Neon tetra disease is sadly untreatable and it is fatal regardless of treatment during any stages of the infection. Due to the nature of the signs of this disease, there are other bacteria that can give the same symptoms and coloration changes to neon tetras that is fully treatable at almost every stage. This other bacteria is commonly called the "false neon tetra disease" due to it appearing to be the original disease.
The early stages of infection of this disease will show no symptoms, or will show very small symptoms such as not eating fully, being less active than their tank mates, and lastly staying on the outside border of their shoal. As the disease progresses, more symptoms will appear including lack of eating, the fish's color may fade (if the fish is a neon tetra the color between their tail and body will turn an off white or gray color while their blue fades), the fish may have trouble swimming around, and lastly the shoal of fish will appear to have "kicked out" the infected fish from their group. Whenever a shoaling or schooling fish is sick, the rest of its species members will remove the infected fish from their group in order to protect them from getting the sickness as well (hence why seeing a lone fish that should be in a group hang out by itself is normally a bad sign to start with).
Pleisotphora hyphessobryconis is the parasite that is the cause for the neon tetra disease and allows for secondary bacterial infections to also infect the fish. The parasite enters the fish's body by being accidental ingested either through a food source, if the fish eats another infected fish's poop, or if there are any nips that the fish may take to other tank mates who are infected. Once the parasite enters the body it starts to produce a highly large number of spores that are able to infect other fish by the same accidental ingestion. However as this parasite makes its way into the organs, it will also release spores into the fish's digestive tract that can infect other fish if they are inside of the tank water column.
Treatment and Medication
Unfortunately there is no known cure to this parasite although many have stated that using a bacterial and fungus medication does slow down and can prevent death if given in the early stages, there is no known documented proof such medicines do truly work. The only way to make sure that you do not get any fish that have this disease is to always buy from a store that is not only reputable, has not had any fish die showing the symptoms listed above, but mainly only gets new fish from trusted and well known breeders.
Due to the way that this disease effects the inhabitants, the best prevention methods are to always make sure that you quarantine anything that you place inside of your water (this includes any plants, inhabitants, decorations, and more). Thus, when you do let them adjust to your water and you truly see how they are acting, eating and if they have any symptoms or signs, we will either start to get a better understanding if they are infected, or learn about what possible other diseases they might have.