The fish itself is the first difference
An ordinary pond goldfish you might move is a modest animal you can cup in two hands. A mature koi is a heavyweight — several kilos of muscle that has often lived in the same pond for years and may be worth a great deal to its owner. That size drives every other difference in this article. A big koi consumes oxygen at a rate a goldfish never approaches, produces waste in proportion to its body mass, and can injure itself against a bag or box with a strength a goldfish simply does not have. Transport that treats a koi like an oversized goldfish fails on all three counts, usually within the first hours of the journey.
Bagging: one fish, more water, pure oxygen
Goldfish travel happily several to a bag of water topped with air for a short trip. Koi go one to a bag — a large, thick-gauge bag, doubled — filled with enough clean pond water to cover the fish generously and support it, then inflated with pure oxygen from a cylinder rather than room air, and sealed. The oxygen headspace is not a luxury: it is what lets a big fish breathe through a journey measured in hours. The sealed bag goes into a rigid box, kept dark and level, cushioned so it cannot roll; darkness calms the fish and a koi that cannot see out spends the journey resting instead of fighting the bag. Never use a rigid bucket with a loose lid for a large koi — sloshing in a hard container batters the fish with every corner and stop.
Fasting and timing are stricter for koi
Both fish travel better on an empty stomach, but with koi the fasting is non-negotiable: withhold food for several days before the move so the bag water stays cleaner for longer, because a koi that feeds normally up to moving day will foul its limited water with ammonia while its gills are trying to breathe from the same supply. Work the journey around the fish — travel in the coolest part of the day, avoid leaving a boxed koi in a parked warm car for even a short break, and keep the total bag time as short as the route honestly allows. A goldfish forgives a dawdling, multi-stop trip in a way a large koi in warm water does not.
Arrival: the acclimation is slower and the water matters more
At the destination, float the sealed bag to equalise temperature, then open it and mix new pond water in gradually before release — and resist any temptation to pour the bag water into the pond, because it arrives loaded with the waste of the journey. Watch a newly moved koi closely over the following days: off its food briefly is normal, but clamped fins, gasping at the surface or lying on the bottom warrants an immediate water test and a call to a koi keeper or aquatic vet if it persists. Because koi are long-lived and individually recognisable, a receiving pond also needs to know which fish is which — pattern, size, any past health notes — so each fish's record travelling in its KinsLe Pet Passport lets the new keeper match fish to history from the moment the bag is opened.