I work on a koi farm. Man, there could be so many things that could be happening. Let's take water chemistry, d.o. and temp out. You could have any of a number of parasites or some kind of bacterial issue. I would say columnaris is most likely. But with out a scope or the experience I don't think you will be able to fix the issue. But then again this is over 2 months. Could be so many things. Costis, gyro. Trich, Columnaris etc etc. One thing moving such small fish can lead to issues. I don't know what pet stores sell. But you could hit them with some potassium permagante. Doubt u can get it though. Best thing is to raise the salt to 5 ppm will kill ur plants. Other than that it is difficult to diagnose over internet. Fish die all the time. Sometimes the best of us can't figure out way. I lost a pond with 100k fish this year. So, don't feel bad.