When your koi grow to above 18 inches then great care must be taken with moving them. In the United Kingdom the most favoured way of doing this is by use of a "stocking net". These are available from all large koi dealers in this country and cost about £16, or approx 25 U.S. dollars.
Recently one of our members built a new pond and then had to move his large fish from his old pond. Several club members went to help and these photographs were taken whilst two of the fish were being moved.

In this photo the platinum oghon has been netted from the pond, and is now being transferred to the koi basket.
You will note that a large shallow net is being used so as not to damage the fish, and the basket is lowered below the water level so that the fish can be floated into the basket without lifting it out of the water.

In this photo the fish is now in the basket, and is being transferred to the net stocking.
The water level in the old pond has been lowered to make it easier to catch the fish.

Here we have a Kohaku that has been but into the basket, and is being inspected before putting into the net stocking.
These baskets are used in Japan for holding fish at auctions, but at that time the fish are usually in plastic bags. In the U.K. we use these containers to hold the fish, but great care must be taken especially with larger fish to ensure that their fins and/or tail are not caught in the slots as this can cause serious damage.

In this instance the Kohaku is being transferred to the net stocking before being transferred to the new pond.

The Kohaku is now being transferred to the new pond.
The net stocking is open at both ends so it is necessary to hold both ends tightly whilst the transfer is taking place.

The net stocking is placed in the new pond, one end is released, and the fish is allowed to swim out.
The temperature of both ponds has been checked and the temperatures were found to be the same so there was no necessity to float the fish in bags which may have been necessary if the new pond had been at a different location.