Internal Parasites
My koi is in a 55gal tank. I have had him over 3yrs. He was given to me by my local pet store owner because they had him for 6 months and he ate all the time and kept wasting away to the point where he was a big head with a skeletal body. I feed him everything from pellet food to earthworms I dig up from outside (his favorite). When I first got him years ago, I treated him for everything and nothing. A friend had the Jungle Internal Parasite Med and I tried 4 courses of the medication and he started eating and gained weight. He looked like my other koi, fat around the belly and tail, until recently and it's starting again. Also, no matter how much I feed him, he never produces any waste. I hope this helps give you a better idea what is wrong with him. One last note, he is pure white and does not have any signs of sores, missing scales, or bugs on his body. He does not rub on the bottom, but he does suck up the tank medium(crushed shells) after he eats, they spits it out. Help!