It is possible that his new arrivals could have parasites or some latent bacterial disease that has yet to rear its ugly head. Here's the scoop. A fish can stress from high ammonia levels, being shipped, netted, from handling, moving to a new environment, or simply from unhealthy water conditions. The fish could stay in this bag from 4 to 24 hours. The fish are then put into holding tanks that still are not the ideal situation for them to be in.
Most tanks are over-crowded and the fish are constantly netted.
Then you purchase your fish and it is rebagged for the trip home. For the koi, the duration of this event could be as long as 4 days. If you have the equipment to do a scraping, then do several. This is the preferred method. This means taking for granted that the fish have parasites.