Fry die off

[kevrod]

Last summer my koi spawned and I was able to preserve 35 or so of the fry. I put them in a fish tank to winter over to increase their chance of survival. I had planned to reintroduce them into the pond once the water was warm enough. They have done great however over the past two weeks a 4 of them have gotten sick and died. They lost their ability to maintain bouancy and often float vertical or even upside down for long periods of time before they die. It started with 1 then another, then a week or so later 2 more. I’m ready to reintroduce the rest to the pond, but fear they may introduce something to the older koi. Help please??? Any suggestions would be greatly appreciated.

Heres a link to my pond if interested…
https://youtu.be/yv0WKK33NkE