Few days ago I bought two Shubunkin and two Koi. All are 3-4" First day they seemed okay, swam around and ate some. Now the two Koi and one Shubunkin (he barely moves, and seems to be breathing heavily) hide all the time, never see them eat. While the other Shubunkin is flourishing, chasing and eating. I only have a liquid ammonia test that reads the lowest it can. I put 2.5kg of pond salt into a 1800l pond. Hopefully help with stress. I don't understand how one new fish is doing great and the others look half dead. All the older fish have no problems and all look healthy, just the new ones.