ribbon eels have a bad survival rate in captivity, as BIGgourami stated, a melatremus eel would be the best, liveaquaria.com sometimes have the in their WYSIWYG section, however they are 300-400 dollars. IMO, a snowflake eel will get to big for that sized aquarium. As tylerperkins stated, you could get a wolf eel, although not a eel at all, it looks like one and grows 18 inches in length, and will run you, at leat in my area 50-75 dollars.