Two things spring to mind: spaghetti eels and true eels.
True eels (
Anguilla spp.) should be avoided at all costs. They are destructive, big, predatory, difficult to keep inside a tank, and ultimately need conditions home aquarists cannot provide. They're hardy, yes, but otherwise of no particular interest.
Spaghetti eels are typically
Moringua raitaborua, a member of the family Moringuidae and essentially a dwarf moray eel in shape. They are very slender, quite small, social, nocturnal, and feed on insect larvae mostly. They make good aquarium fish, though they are shy. Moringua raitaborua at least does well in low- to mid-salinity brackish water tanks with a sandy substrate for burrowing. Keep in groups, and feed things like bloodworms. Not common in the trade, but definitely worth snapping up if you see some.
How to tell them apart? Spaghetti eels lack pectoral fins; true eels have well developed pectoral fins. Spaghetti eels also have a wide, puffy-looking gill chamber at the throat, similar to that of a moray; true eels don't have this chamber, and the throat is similar looking to the rest of the trunk. Finally, spaghetti eels have weakly developed dorsal, anal, and tail fins, giving them a very worm-like appearance; true eels have well developed anal, dorsal, and tail fins and an overall powerful, stocky build.
Cheers, Neale