Why are Koi carp such delicate creatures?

I had about 30 Koi carp in a pond in my back garden. Now my Weimaraner Rufus would occasionally gobble one of them up, which I didn't mind as the stock would replenish themselves. However, I recently held a garden party to celebrate my promotion at Goldman Sachs, it was a lavish affair, champagne and ferrero rocher - we wanted for nothing. Unfortunately as the night wore on my guests became 'worse for wear' and started to use my pond as a toilet. I woke up the next morning and all my fish were dead. I am deeply saddened by this as they will cost a fortune to replace - is it usual for these fish to be killed off by such a minor thing?