Many years ago my girlfriend gifted me a "freshwater moray eel". I was delighted of course, and it did well at first, then it started not doing so well. I gave it to a friend who had a brackish set up and his health improved, with behavior returning to normal. This was a long time ago so I cannot remember the exact species besides being labelled "snowflake eel". This could have been a wrong label as the same girlfriend also surprised with me a very young "piranha" which turned out to be a Pacu, from the same store.
From my research at the time, it looked like there was no real "freshwater morays". They migrate to breed, and live as juveniles like some other marine/brackish fish do, but dont survive in freshwater for life.
That being said, I am no expert on eels, but I found this link on Gymnothorax polyuranodon - freshwater eels. They seem to be found in fresh and brackish. Although not much research has been done on them by the looks of it, so their habits and patterns are not well known yet.
Granted the paper is from 2011, but I didn't find much else on them.
<p>Analysis of 36 records of the rarely encountered moray Gymnothorax polyuranodon indicate that juveniles and adults inhabit fresh and mildly brackish habitats (salinity < 5) in streams of the Australian Wet Tropics Eighty-one per cent of these records were from freshwater streams and...
www.ncbi.nlm.nih.gov