moving goldfish from pond to aquarium

I have two oranda goldfish that i started out in a 24 gal tank together. the problem is one is a wen picker and i thought it was because the tank was too small. i moved one of the orandas to an outdoor pond, which it has done well. the temp here gets too cold in the winter to leave an oranda outdoors(the pond will freeze over). I have brought the fish back indoors and placed it in a 54 gal corner tank with the same oranda as before. same thing , it is picking at it's wen. any idea if it is okay to bring a fish in from a pond? the temps are the same now but im not sure about diseases and soforth. Should i trade this fish back in and keep the one oranda by itself since it picks so bad?