Wow - I'm surprised the mollies made it from fresh to saltwater like that. Even with long acclimation (over days), it is pretty common to lose some of them in the process.
Here is an essay I wrote on the black molly method:
Using black mollies to screen an aquarium for Cryptocaryon
In recent years, a procedure has been promoted that uses marine-adapted freshwater mollies to screen for active Cryptocaryon infections in marine aquariums. The thought is that black mollies that are naïve to marine ectoparasites, will soon develop infections if the disease is present in the aquarium. Being black, these parasites will show up in sharp contrast, making their identification much easier. Mollies have also been suggested to be housed alongside sensitive fish (that cannot be easily medicated) to serve as a “canary in the coal mine” for active disease.
The basic process is to acquire a small group of freshwater black mollies and gradually acclimate them to seawater over a period of 5 to 7 days. They then are added to the previously fallow aquarium, or added to the quarantine tank, and then observed for at least two weeks to see if they develop ectoparasites. If they do, then a treatment needs to be instituted, or the fallow period extended.
As with many aquarium ideas, over-extrapolation can reduce the effectiveness of the original idea. The process is really only suited to screen for Cryptocaryon. Brooklynella may not even infect mollies. Uronema and Amyloodinium can survive salinities as low as 3 ppt so may already be present in “freshwater” mollies that have been raised in brackish fish ponds. Marine and freshwater fish have basically the same internal salinity. Therefore, untreatable internal diseases, such as viruses and Myxozoans could possibly be brought into an aquarium with the mollies.
There is a risk for introducing euryhaline trematodes into an aquarium along with black mollies. Fish farmers, wholesalers and retail dealers all understand that mollies benefit from being housed in brackish water, and so they usually add salt to systems housing mollies in order to reduce mortality under crowded conditions. Euryhaline trematodes take advantage of this, the trouble is that some of these can survive marine conditions and then hyposalinity is ineffective as a treatment for them.
Finally, here have been no scientific studies that indicate this method is actually effective, it is based on sound theory, but needs to be better tested. This process does seem to have benefit in screening for Cryptocaryon, but falls short for other diseases.
Jay
