Depends on what kind of "snowflake eel" you have.
If you have a G. Tile "Freshwater snowflake moray" then I'm actually not sure if its a good idea to keep it in marine conditions perminantly, but I'm not positive on that.
However, if you have a saltwater Snowflake, then basically raising the salinity .002/weekly with your water changes - until you reach your desired level is what you'll wanna do.