definatly pass on the ribbon eel. they are very cool but almost impossible to keep.
30 gal would work for a snowflake eel (which is my favorite)...i kept mine in a 30 gal. for 2 years, but after i moved him to a 75 a few months ago he seems much happier and seems to be growing faster too.
If you do put him in a 30 i would hope that you are prepared to get him a bigger home in 1-2 years.
hth,
mike