With a temporary quarantine tank outside of your pond you will need to purchase several items. Here is your list.
• A container to use as your quarantine tank. You have the option of using a children's swimming pool. They can be purchased very inexpensively at the end of the summer or at flea markets. One item that could be used is a plastic tub such as the Rubbermaid ones you use for storage of items. If the fish are small you could use a large aquarium. The other option is horse or cattle troughs. Do not use anything made with a Latex sealant or metal because they could be toxic to your fish.
• An aerator is required. You can purchase an aerator kit but not one for an aquarium. Another option is to purchase a pump with a fountain or hose.
• Biological filter is recommended to cut down on the number of water changes. The mechanical filter removes solids and the biological filter removes ammonia and nitrites. You could purchase a kit with a filter, pump and fountain. Make sure the pump will circulate the water every 30 minutes.
• Your Koi will jump, especially when in a new environment so a cover is needed for your temporary tank. Netting, plywood, or a window screen can be used. Be sure to weigh down what you use because the larger fish can lift the cover. If you are using something solid don't forget to leave a hole for the gasses to escape.
• A heater is optional.
The quarantine tank is a mini version of your pond so you will set it up the same way. If you use a submersible pump be sure to put it in a net bag. You don't want the Koi to be hurt.
The size of the tank depends on the number of Koi and their size. You will want it to be up to 300 gallons. When you start going larger you should think about a permanent tank. Also, the bigger the tank the more it costs to treat it and the harder it is to catch the fish.
To start your tank you can use water from your existing pond. It is best to set the tank up several months before you use it in order to build up the bacteria that is needed when the fish are added. Once the tank is ready and your fish are settled in you will need to make water changes. With the use of a filtration system a 10-15% water change is required a couple times a week. If you opted to go without filtration then you will change 50% of the water daily. Check the water for ammonia and nitrite changes. If you are not feeding your fish then the levels are easier to control. Your fish can go at least 2 weeks without food.