I have had best of both worlds dealing with Fire eels, i bought one at about 10 inches long and he is obsessed with eating night crawlers so far over the past 3 months he has grown about 2 inches in a 125 gallon tank. I also have a second fire eel that i raised from 5-6 inches long, it eats frozen bloodworms about two times a week and night crawlers 1 day a week, this second one has grown a good 5 inches long and has fattened up quite a bit alone in a 55 gallon tank. Average if fed well a small fire eel should grow about 1 inch per 2weeks-1month