In the long run I would say that, that snowflake eel will be much too large for a 20g anyway. I have a 2' snowflake in a 125g with clowns but, they host a anemone and are on opposite ends of the tank. And I also totally agree with FLOWER. When full grown they will have a bigger appetite and may go after the smaller fish in the tank especially in a small tank. I would recommend at least a 50g for the snowflake. Good 20g fish would be like gobys, fireball angel, green chromis, black bar chromis, neon goby, jawfish, scooter blenny, basslets, damsels, ocellaris clowns think they my even have a small puffer that would be alright in a 20.