Goldfish need more water per fish than any bag suggests
A goldfish travels safely only with far more water and air than its size alone implies. Goldfish eat heavily, breathe heavily, and release waste steadily, so a snug bag that would suit a tetra turns foul quickly around a goldfish, especially a large fancy variety. Use strong double bags filled about one third with tank water and two thirds with air, or a rigid lidded tub for a big fish, and give each large goldfish its own container. Crowding two adults into one bag to save space is the decision that most often turns a routine goldfish move into an emergency.
Handling a heavy, round-bodied fish
Lift a goldfish by supporting its whole body, never by scooping it dangling in a net. Fancy goldfish carry deep, rounded bodies on delicate fins, and a fish lifted by a net alone can strain its body, tear finnage, or aggravate balance problems it already manages in the tank. Slide a smooth container under the fish in the water and lift fish and water together. Stop feeding at least a day before the move, because a goldfish travelling on a full stomach fouls its limited water much faster, and the cleaner the travel water starts, the longer it stays safe.
Packing the life-support, not just the fish
A goldfish's filter carries the bacteria that handle its heavy waste, so that media is as important as the fish on moving day. Pack the media submerged in tank water, keep sponges and ceramics from drying out in transit, and move the fish and media in the same trip whenever you can. Fill travel containers from the goldfish's own tank rather than fresh tap water, so temperature and chemistry match what the fish is used to. Set the containers upright in a dark, insulated box, wedged so they cannot roll, because a sloshing, light-struck journey exhausts a fish that cannot brace itself.
On the road with a waste-heavy fish
Keep goldfish water cool and shaded in the car, and resist opening containers to check on the fish. A sealed bag holds a balance that changes once opened: fresh air alters the water and can make the waste already in the bag more harmful, so a well-packed bag is best left sealed until you are standing at the new tank. For journeys of several hours, rigid tubs with a battery airstone are the safer setup, giving steady oxygen without opening and closing. Plan the route so the fish are loaded last at the old house and unloaded first at the new one, ahead of furniture and boxes.
Into the new tank, and what to watch
Have the receiving tank filled, treated, and running before the goldfish arrive if the move allows, because these fish cannot wait in bags while a tank is assembled. Float the bag or ease container water toward the new temperature, add tank water gradually, then net or tip each fish in and discard the travel water, which now carries the journey's waste. Feed sparingly for the first couple of days while the filter settles. Watch for a goldfish sitting on the bottom, gasping at the surface, listing to one side, or showing red streaks in the fins, and seek advice from an aquatic veterinarian if those signs persist beyond the first day. Carrying the goldfish's size, variety, and water notes in its KinsLe Pet Passport keeps that record in one packet for whoever sets up the new tank.