lol
So the wife was in on it after all. Oh well if I were the husband then I guess I'd just join the island murder cult. not worth upsetting my beloved wife or endangering our daughter.
Aren't they able to kill people a little too quickly? given how small koi's mouthes are, as long as they can't bite the neck, it would take a couple of minutes for a person to die from bloodloss due to a bunch of small chunks being bitten off. Enough time to get to the other side or at least struggle for a while. If they all jumped in at once and swam to different points of the circle then they've got a good chance of reaching to grab the ledge and pull themselves out , and just the shrine maiden and two male helpers wouldn't be able to keep them all from getting out.