Do koi eat algae

Yes Koi are omnivourous. Any meat you give them needs to have all fat removed and be cut into pieces small enough for them to swallow easily. Raw beef and chicken are good. Koi will also eat cheese. Earthworms are a special favourite. Always remove any leftover/uneaten meats within 30 minutes. Failure to do this will result in the water being poisoned and the fish being damaged or even dying.