Always quarantine new fish. Never introduce them to your existing fish until after a quarantine period of 2 to 3 weeks, longer if possible.
Always treat new fish as if they have parasites, viral, fungal, and or bacterial infections. Better to be safe than sorry.
It is also possible that your existing fish have parasites, viral, fungal and or bacterial infections. They may have built up a tolerance, while the new arrivals having undergone the stress of shipment could succumb immediately.
Even if you never plan to "Show" your fish, it is essential to establish a quarantine system for proper Koi husbandry. The Show Tanks we feature, make an excellent Quarantine tank for treating current stock, or especially when adding new fish to your collection.