Snowflake Eel

What size tank is best for a full grown Snowflake Eel?  I currently have him
in a 20 gallon, but plan to turn my 50 gallon freshwater into a salt tank
soon.  I was told over the weekend that even that was much to small.  Also,
at full size, would it be safe to keep clowns, and similar sized fish in the
same tank?  If not my plan is to go with the eel, a yellow tang, and a
lionfish.

Thanks guys.
Matt