fish health in the spring

As noted in my pond books, the fish struggle with poor health in the spring.  I added pond salt this year to help and kept a close eye on things, adding medicines as I spotted fin shredding and white spots.  In spite of these efforts, the large white koi would not heal and I had to remove her and put her down.  She was very unhealthy at the end of last winter as well.  By the time I made the decision to remove her this year she would barely move and was sort of beached in the shallowest part of the pond.  Only a couple other fish were afflicted and they quickly healed up. It seemed like the prettier the fish, the worse their health; none of the non-colored goldfish were affected at all.