I accidentally bought a Koi fish thinking it was just a normal goldfish. After about 4 years now, he has gotten bigger (mouth to tail ~ 7 inches long) and more aggressive in our 30 gallon aquarium. He is trying to eat other smaller goldfishes. That includes thrashing around tank's plants, ornaments, gravel, etc... Sometimes he bangs into the glass making very loud noises. We feed him more food than the amount of food small goldfishes eat. We all like him, but we are at a loss as to how to deal with him. Should we take him out of the aquarium and put him in a separate container a lot bigger than 30 gallon? or simply give him away?