I have never seen a koi or goldfish spawn which did not get fungus. Nor have I ever seen a koi or goldfish spawn where the fungus killed
all of them. Without treatment, 10-50% seems to be the norm. The warmer and cleaner the water, the less fungus mortality there will be.
Chemotherapeutics never come without a cost. They might reduce the amount of fungus, but increase the probability of a defromed fry. In a normal koi spawn, there are enough eggs to stock a about a thousand tons of water. You do not need to save them all to raise a few in a tank.
-stevehop