Congratulations on your new koi! I am sure it will bring you many years of enjoyment.
Now that you have purchased your new "Living Jewel" we need to make sure it is cared for properly. Koi are not found in nature and due to the selective breeding process they are not as hardy or strong as their native wild cousins. So we need to pay special attention during this very vulnerable time. After being transported, to say the koi are stressed is a huge understatement. They are a nervous wreck and have shed their natural protective layer, the slime coat. The slime coat acts as a natural defense against parasites and bacteria. Without the slime coat, bacteria can attack the koi and it can become sick and die very quickly. That is why you MUST quarantine your new fish before putting it into the pond. Any dealer that tells you not to quarantine is trying to sell you more fish after the ones you bought die. The quarantine process will allow the fish to re-grow its much needed slime coat before it is subjected to the large amounts of bacteria and parasites in your pond...and they are in EVERY pond. It will also allow you to monitor the koi for any possible health or stress related problems in an up close, easy to treat situation. Once released into the pond they will be much harder to catch and treat.
To quarantine your new koi please follow the steps below.
1) Set up the quarantine before the fish arrives. An area with a stable temperature is best, like a garage or basement. 150 to 300 gallons will be fine. You can use a stock tank, show tank or even a kiddy pool. You will need a small submersible pump, apx 500-600 gph and a small filter, preferably with cycled biomaterial. (Without a cycled bio-filter you will need to do more frequent water changes, perhaps daily.) An air pump w/ an air stone would also be a good investment, but not required unless water temps are above 75 degrees. You should also net or cover the pond as koi tend to jump when put into new places.
2) Add 1 pound of non-iodized salt per 100 gallons to achieve .12% saline. (Salt available at Lowe's, Home Depot, etc in 40 lb bags, look for bags that say 99.5% pure salt)
3) Float the bag in the quarantine for at least 30 min.
4) Remove the koi from the bag and release into the tank. It is VERY important that none of the bag water is allowed into the tank. (tip: you can put the koi and the water into a separate bowl, then lift the koi by hand and gently release into the tank.
5) Monitor the water quality daily. There should be 0 Ammonia, 0 Nitrites and low nitrates. If you test any ammonia or nitrites, do a partial water change (don't forget to dechlor!) It may be necessary to add an ammonia binder like Amquel or Chloram-X. Also check pH and kH. You want a high kH to stabilize the pH. If your kH is low (below 80) add a couple tablespoons of Sodium Bicarbonate (baking soda) then test again after a couple hours.
6) Leave the koi in the quarantine for AT LEAST two weeks. 4 or even 6 weeks is better. If no problems arise then it is safe to transport the koi into the pond. If the temp between the tank and the pond is less the 4 degrees different, you can simply release directly into the pond. If there is more then a 4 degree difference you will need to float the koi before releasing. Since the koi is not being transported you can use a heavy-duty trash bag filled with a couple gallons of water from the quarantine tank. Trap as much air as you can in the bag and tie the top in a knot. Then float the bag for 30 min. This time it is ok to simply dump the bag water into the pond since the fish has not been in that water very long.
Congratulations! You have just successfully quarantined your koi!