I have a 150 gallon tank with 2 small/medium Koi (about 4-5 inches long) and 3 goldfish one of which was a fancy tail..until this morning. I caught my Koi biting the tail off of it and now it is missing a large portion of its tail fin. I have since then moved it and 2 Gourami to my sons 20 gallon tank in his bedroom. Will it grow back? and what do I do to keep it healthy? Please give me info I am a beginner .