CFC,
Some interesting stuff in your post. Never heard about their skin "glowing".
There are some curious factoids on these eels if you check out their Fishbase entries.
Gymnothorax polyuranodon for example is said to be a catadromous species, that is, while adults may inhabit brackish or fresh waters for extended periods, they return to the sea to breed. This agrees with Schafer's records of these fish being found 'a long way upstream in completely fresh water'.
Gymnothorax tile on the other hand is *suggested* to be anadromous, meaning that it inhabits brackish and marine waters as an adult but breeds in brackish or fresh water.
Echidna rhodochilus isn't referred to as either, so by implication may be euryhaline, swimming and breeding wherever it wants.
It would be an interesting project to breed these fish. People doing this sort of thing with gobies that swim between rivers and the sea have proved it can be done, but there's clearly a lot we don't know about the biology of these eels. For one thing, if some of these fish breed in freshwater, do they have a leptocephalus larval form like most other eels?
Anyway, thought you'd be interested / have comments.
Cheers,
Neale
PS -- this Fishbase links...
Echidna rhodochilus
Gymnothorax tile
Gymnothorax polyuranodon