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Choose plastic bags designed specifically for transporting fish—they should have flat bottoms when filled and be at least 3 mils thick (and ideally 4 or more mils). Likewise, use fish shipping containers that have sturdy cardboard outer boxes and inner Styrofoam boxes that fit inside perfectly.  You can order these online from companies that deal in exotic fish supplies. You might also find them in fish supply or pet stores. The bags and boxes come in various sizes. If you’re putting a single fish in each bag—which is the safest method—choose bags that are about 3 times as wide as your fish breed’s average length. Choose the box size based on the number of bags you intend to ship at a time. Your chosen shipper might not absolutely require fish-specific bags and boxes, but they’re your best bet for getting your fish to their destination. Move the fish you intend to ship to a separate tank that’s set at their ideal water temperature. Use an aerator and filter to keep the water clean, oxygen-rich, and conditioned properly based on the fish breed’s needs.  Do not feed the fish while they’re in isolation. You want them to create as little waste as possible during their journey. Check for any signs of disease during this time, since you don't want to ship out diseased fish—especially to a paying customer. During the 24-48 hour isolation period, incrementally reduce the water temperature from the ideal to the lower healthy limit for that fish breed. Turn down the tank’s heater a little more every few hours and check the progress with a tank thermometer.  This process will slow their metabolism, which means they’ll consume less oxygen and produce less carbon dioxide and waste. You might think you should increase the temperature to make up for heat loss during shipping, but the box’s insulation—along with heating pads, if needed—will take care of that.

Summary:
Buy shipping bags and boxes designed for fish transport. Place your fish in an isolation tank for 24-48 hours without feeding them. Decrease the water temperature gradually to your fish breed’s lower limit.