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.
Also these tanks, in conjunction with a small heater, are ideal for growing smaller fish.
This widely used quarantine system from Koi Carp, unlike other systems, simply is in our opinion the best on the market. It comes complete with grid or dome with built in 110 mm bottom drain an essential part for any quarantine tank or system set-up incorporated by using 4 purposely built legs.