i have a fire eel in my 55 gallon,but it lived it's first year in a 20 gallon,and was more content in the 20 gallon in my opinion.it's been in the 55 gallon for about 2 months and hides away all the time.in the 20 gallon,it was out and about all the time,plus,the 20 gallon was full of small neons and it never ate 1 fish,so don't always follow the rules,sometimes,personel experience is much better.