The following is taken from the thread on freshwater and saltwater diseases Neon Tetra Disease (Pleistophora hyphessobryconis)
Restlessness especially at night (a weird behavior can be distinguished when the afflicted fish seems to be swimming on its own while the rest are "asleep"), loss of coloration on its body usually starting on the midsection, secondary infections brought by finrot and dropsy, curved spine, erratic behavior Causes:
This is caused sporozoan parasites and is very contagious. Treatment:
There is no known cure up to this date as the sporozoan parasites have proven themselves very resistant to several medications. Any fish suspected to be afflicted or afflicted must be isolated immediately
and eventually euthanized if no improvement in its health has been observed in the next few days. Transmission is done mostly by fish picking on the afflicted and spores infecting new hosts as the current host dies. Notes:
This disease has been confused with Columnaris. Note that the NTD is usually associated with a creamy colored patch which appears to "delete" the actual coloration of the fish. Columnaris is associated with white patches, a case very different from NTD.
Neons are very vulnerable to NTD but this is also found among cichlids, danios, rasboras and other species. There have been several reports that so far only the cardinal tetras are immune to this disease.
Read more: Freshwater and Saltwater Diseases