I've got 3 weeks left on fallow display until my fish can go back in. I was thinking about adding a black molly or two then to the sump to see if they show signs of ich before adding my saltwater fish just as an extra precaution . Some people have said acclimating them to salt isn't easy but wondering what consensus is? Also I am assuming freshwater ich already on a molly would not survive in the salt to be confused with marine ich?
Regarding the molly idea - that is one of those things that got over-extrapolated. It is a nice theory, but in practice, it only works in limited instances. Here is a write-up I did on that:
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 freshwater black mollies that are naïve to marine ectoparasites, will soon develop infections if that disease is present in a marine aquarium.
With mollies being stark black, white 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 up to 5 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 fishponds. 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 also 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, black mollies have been used by scientists to propagate Cryptocaryon in laboratories. However, these studies went no further and there is no scientific evidence to indicate this method is actually effective. It is based on a theory, but it needs to be better tested. If properly applied, this process may have some benefit in screening for Cryptocaryon but falls short for all other diseases.
