In northern locations with cold and sometimes extreme winters, some people opt to bring their koi indoors for the winter. Although koi and goldfish can survive harsh winter conditions, if you have relatively few koi, or if you have a large tank to house your fish inside for the winter, you might want to consider bringing your fish inside. Here are a few pointers and tips to help make the transition from the pond to an indoor tank less stressful.

Pros:

• Fish are protected from extreme cold and temperature fluctuations
• You can closely monitor water quality and fish health
• You get a chance to inspect the fish closely while catching them from the pond
• The fish will continue to grow during the winter
• You get to enjoy your fish year round

Cons:

• Indoor tanks require a good deal of maintenance ie. feeding, water changes, filter cleaning etc
• The fish are stress and could be injured from being caught – twice (fall and spring)
• Usually requires extra equipment, pump, filter etc
• Water quality problems  are common, especially during the first month which often lead to illness or death.
• Koi are jumpers – you must make sure you tub is covered very well particularly for the first few weeks
• The fish will continue to grow over the winter which isn’t good if you have limited space
• You will most likely need to drain your pond completely to catch fish

If you have weighed the benefits and drawbacks of moving your fish indoors for the winter, and want to go ahead with it, here a few pointers to follow to help make the transition a success.

• If possible, set up the filter you are going to use for your winter tank in the pond and let it run for at least a month prior to bringing your fish inside. Then move it inside when you bring your fish in. This will help to ‘cycle’ the filter and seed it with beneficial bacteria, reducing water quality problems that usually occur when starting a new system.
• Set up the tank, install the filter etc & fill it with water from pond. Check all connections, tubing etc for leaks.
• The easiest and least stressful way to catch your fish, is to drain the pond leaving just enough water for the fish to swim. Catch the fish, gently place them in bags or containers and bring them into the house. If you have large koi, a Koi Sock Net is the safest way move the koi from the pond into the bag or container you are temporarily holding them in.
• If you have just filled the tub with pond water, there is no need to float the bag to acclimatize the fish, just gently release them.  However, if the tub has been set up and running for a few days, float the bags in the tubs or tank to let the temperatures equilibrate.
• Once all the fish are in the new tub, make sure it is covered very well. Koi are very good jumpers and will jump, especially for the first few weeks.

• Add Beneficial Bacteria such as Microbe Lift PL , or Aquscape Dry Bacteria to help seed filters and improve water quality.
• Feed sparingly with a quality Fish Food for the first few weeks, this will help to reduce ammonia and nitrite spikes in the water. Some people prefer to feed a Spring & Fall wheat germ food because it is very digestible and the fish produce less waste.
• CHECK WATER QUALITY with a Test Kit. I can’t stress this point enough. At the very least, test for pH, ammonia, nitrite. Most fish illness and deaths occur in the first month, and it is almost always as a result of poor water. Filters take time to become fully active or ‘cycled’ with the beneficial bacteria that break down the waste that fish produce. Until the filter is cycled, ammonia and nitrite can rise to toxic levels quite quickly, which is very dangerous to fish.
• Make sure to perform partial water changes regularly. This does not mean ‘filling the tank’ when the water level drops from evaporation. You should drain 10% -20% of the water and replace it every 1 to 2 weeks (remember to add Dechlorinator if you are on town water).  You may need to increase the frequency and the volume during the first month if you have high ammonia or nitrites, or if you have a high fish load.
• As your water quality stabilizes, you can gradually increase the amount of food you feed your fish. However, please realize, that the more you feed, the more filter cleaning and water changes you will need to perform. 
• Enjoy your fish.

Returning your fish to the pond is usually a much easier process. There are just a few things to consider when moving them back outside.

Steps To Follow:

1. Clean the pond, remove any sludge and debris that has accumulated over the winter, perform a partial water change and install pumps, filters etc.
2. Make sure your pond is running at least a week before  putting your fish back in the pond to make sure everything is operating properly.
3. Don’t move your fish outside too early. Make sure the pond has warmed up and is less than 10°F  cooler than the inside tank. Too sudden a drop in temperature is stressful on the fish and can trigger parasite outbreaks.
4. Once the fish are in the pond, monitor them for signs of stress and check water quality.
5. Feed the fish a proper diet according to the water temperature.