Why is my fish eating my other fish all of a sudden?

We have a large tank, I'm not sure how big but at least 15 gallons with 8 inhabitants. Two are black moors, two are koi, 3 more are also goldfish (not sure on what kinds), and one is a algae eater. All of the fish have been doing fine and have lived together for two years with the exception of the Koi which we added last year. One of our black moors has always been the smallest in the tank because she came to us with a deformed fin or possibly broken either way she's always been the slowest and smallest but none of the fish have ever bothered her. Our biggest fish is the female Koi and she about doubles the small black moor. We also recently switched food but the fish seem to like it just as much as the old kind. Well this morning lo and behold I wake up to our female Koi attacking and eating the small black moor. She no longer has any tail fin at all and I am not sure if she will make it we are putting her into a hospital tank but I don't know if it will help. Are the fish not getting enough food or is it maybe another reason? Will it be safe to return the smaller black moor to the tank when she is healed, if she heals?