I realise that you essentially have your answer, but just thought I'd add:

Koi are opportunistic omnivores, and will eat almost anything they can get in their mouths. Actually, they almost vacuum-feed, sucking up whatever's in front of them and shooshing out anything inedible. I wouldn't put *anything* in a koi pond, except other koi.

(Koi are an introduced pest where I live, and they destroy native species and habitats, so we have plenty of information available on their Dark and Nefarious Ways.) *L*