Not good for those folks but good for you! Are any in the video 36"? If yes, which ones?
IIRC, your filtration and pump size were not intended to handle such a large bio-load. Please, be vigilant and consider increasing both the filtration and the water turnover - koi need both, they are messy, heavy fish, some of the top ammonia producers. I'd guesstimate you need a ~3x turnover, which at 4600 gal would be ~15,000 GPH. I'd recommend three AquaClear 5000 GPH pumps, 360 Watts each, roughly ~$300-$400 each. Two at the very least, with no margin for error then.
Until you get there, I'd feed very, very, very (!) conservatively.
Test for ammonia frequently and at the worst perceived spots. If you see any at all, your filtration is not coping.