Pond Trouble HELP
 

About a month ago, 3 Koi were added to my pond, these fish were added because a family members pond was damaged by a tornado. Before these three fish , I had small Koi and Minnows in my pond. Just recently all my small fish including 2 of the 3 Koi have died. I drained 25% of the pond and added new water ( along with Chlorine balancer )
I took the other big Koi out (he is going to die soon, he looks pretty rough)
All thats in my pond now are minnows and 2 small goldfish ( which I am pretty sure will die soon as well) I tested the water and everything is in balance.

Anyone have suggestions on what I should do before I add new fish. or have a clue why they might be dieing?