My firend just gave me her 2 1/2 inch goldfish and he is alone, just him. The tank she gave me looks like it is 1-2 gallons big. The fish seems happy, but I am worried he is in a too small tank. Theres a small fake plant in there and colorful gravel. Is this enough for him? How big should his tank be if he is just over 2 inches big and its just him. Also, I clean his tank twice a week, once monday and once wed.

the tank he is in will last him a couple months, but in order for him to grow to his full size without dying prematurely, he will need at least 15-20 gallons for him alone