When fish are shipped from place to place by air, they are packed in sick plastic bags with pure oxygen, usually an ice pack. By the time they arrive the water is in poor condition. With a lot of ammonia and a high carbon dioxide levels and a low pH.
The fish have been retaining ammonia for at least as long as they have been shipped, they are hungry and they are stressed.
Acclimating the fish across the wide range of temperatures takes various amounts of time and represents different levels of stress. A 5° change in temperature is virtually irrelevant. Changing 20° in a short period of time will often kill.
When you open the bag, after a 20 minute acclimation period, you need to spend five minutes mixing pond water with the shipping bag water. When you have 50-50 ratios of new and old water, you net the fish out of the bag into the system. Don’t dump shipping bag water into your main system.
Ostensibly, that is a quarantine system.
