Spiny eels should only be kept in very slightly brackish water (SG up to 1.005).
All moray eels do well in mid-brackish to marine (SG 1.010 upwards).
Spaghetti eels need either brackish or marine conditions depending on the species, but the commonest species, Moringua raitaborua, should be kept in low to moderately brackish water (SG 1.005 to 1.010).
If you're running a tank with things like monos and scats, then you can maintain the aquarium at SG 1.018 and keep marine species like snowflake morays without any problems at all. Most marine fish thrive at slightly below normal marine conditions, a fact that was exploited in the early years of marine fishkeeping for various practical reasons.
Cheers,
Neale