I've wanted one of these for years, but don't really have the money right now to finish my tank set up. Based on my research which included the previous posted paper, G. polyuranodon is a true euryhaline species* with a catadromous life cycle, unlike its congener G. tile which is a brackish species commonly sold as a "freshwater" moray. There are even reputably populations of this species in isolated lakes, indicating they are in fact capable of breeding in FW.**
I heard it suggested by the local expert on brackish morays that it might be beneficial for the fish's immunity to establish a "cycle" of fluctuating salinity similar to what would happen in the lower reaches of an actual river. He does it by mixing in varying amounts of seawater with his tap water. My own personal plan was to keep it in a planted Australian Wet Tropics Biotope that matches the native water conditions of the eel and cycle between 1.000 and 1.005 SG over the course of a 2 month cycle via water changes supplemented with marine salt. However that may be overkill, as I've heard of plenty of MFKers and others keeping these fellas in freshwater long term without health issues.
* that's not the same thing as a freshwater species, which would imply it can only tolerate FW conditions.
** In practice, raising moray elvers is almost impossible, which is a damn shame given that morays are well adapted to tank life in most other respects.
