Koi fish today are not the same tough fish as their ancestor, the magoi carp. The magoi carp are extremely resilient fish. They can survive in very poor water conditions and host a serious parasite population without getting sick. Because our modern day koi have been selectively inbreed, so much, to produce the beautiful colors and shape that we all love, they have lost much of their ability to endure such harsh treatment. Still, today's koi seldom get sick and or die without a reason.