Tetrahymena pyriformis disease
The introduction of un-quarantined fish in to the aquarium. Tetrahymena is one of the background organisms found in aquaria, it is usually harmless unless the fish are stressed or it reaches much higher than normal background levels, in both cases it may become pathogenic.
Curiously and unlike white spot, some fish are extremely susceptible to this disease and could die in the relatively early stages of the infection while other fish in the same tank remain completely free of the infection. This is one of the things to look out for when this disease is suspected.
Note - it is most unlikely that a fish will exhibit ALL of the above possible symptoms.
Treat using a proprietary white spot remedy but the infectious agent isn't particularly susceptible to most remedies and so in order to effect a cure you may need to repeat the course of treatment more than once. Which in itself can be stressful to the fish.
Even with rapid treatment you should expect some fairly fast and quite dramatic losses with this disease. Make sure that carbon or any other chemical filter media is removed from the system before treating and don't make any water changes during treatment.
Despite the common name of guppy disease this condition can infect a wide variety of fishes although it is more common in livebearers. Superficially the condition could appear to be white spot, but there are some important differences the main one being that if you raise the temp in an effort to help treat Tetrahymena you will only succeed in making things much worse because the parasite will simply reproduce at a faster rate and prove to be even more lethal. This is another reason why we don't recommend the temperature method for dealing with white spot infections.