I'm thinking the ones you see are actually Gymnothorax tile, the "freshwater moray", as I have an adult, but the only specimen I saw looked a little different (juvie of course), so I couldn't be sure it was the same thing.
I have had mine if full marine for two years now, and I'm not sure how many years or months it was kept in salt before that. Seems to be thriving.
As for keeping it in freshwater, read the following:
http://www.wetwebmedia.com/ca/volume...water_eels.htm