Bitten in half? It sounds like something else bit your goldfish. Something
with teeth or a beak that can sever. It's probably better to not mix fish
of such different sizes together. How large is this tank and is it
outdoors?
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the tank is 4 ft and indoors and there was only the koi and the goldfish in there, so it was definatly them. ive just got to remember never to do that again lol.

cheers for your input