From all that I have read, they will enter into freshwater areas in nature, but if you want them to thrive there must be some rain salt present in the water. I would suggest a specific gravity of 1.005, which is very low end brackish, and many freshwater plants and fish will tolerate this salinity. I have ordered a Antennarius biocellatus, which is an estuarian fish. There is a difference between living in freshwater, and really thriving. The same can be said for "freshwater" Moray eels. There are true freshwater pipefish, that do not require any amount of salt at all.