He is probable eating the food that is left on the bottom of tank,some are real shy and only come out at night when there is not any thing going on and it is nice and quiet and the lights are off. I just got rid of a 14 inch fire eel that I had in my 420,because he never swam around. He was nice and healthy but never colored up,so I traded it for a small bowfin.I have a tire track eel and another fire eel that come out all the time and they love the frozen blood worms,freeze dried krill and pellets ,the tire track eel is really thick about 14 inches.I also have a spotted eel that never comes out,it looks like a big worm compared to the other ones.I keep because its kind of cool looking but came very close to saying bye bye