Fish and plants need very different things in the winter, but can be kept in top condition and health for the following season if the appropriate steps are taken.  

When the water temperature falls below 50 degrees, or whenever the fish start to lose interest in food, feeding should be eliminated.   Not only do goldfish and KOI not need to eat during the winter, it can actually be bad for them to be fed. 

Fish depend on certain enzymes and bacteria in their digestive tract to break down fish food.  These enzymes and bacteria thrive in warmer months, but start to decrease substantially when the water temperatures start to drop.  

This means that food can pass through the fish's digestive tract undigested, and potentially cause blockages or start to decay inside the fish.  This can also be promote bacterial infections.  So, for these reasons, do yourself (and your fish) a favor and resist the urge to keep feeding them.  

Although wheat germ foods and special spring / autumn foods are formulated for easy digestion, they should only be fed in semi-cool water temperatures, and should also be stopped in water temperatures below 50 degrees.  The fish have plenty of fat stored up in their body from their summer-long feast to last them through until spring.

The other important element in keeping the fish healthy over the winter is to provide adequate gas exchange in the pond.  This means providing an outlet for toxic gasses to escape as organic pond debris like leaves and plants start to decay.  

This also means allowing a way for oxygen to enter the water for the fish.   This can be done simply by preventing the surface of the water from freezing over completely.   We suggest using a pond deicer to do this.   These are easy to use, just plug it in and drop it in.  Ideally, pond owners would also add an air pump to provide adequate oxygen. 

Some pond owners like to leave their pump / filter running during the winter, but we recommend that the pump and filter be shut off.  

There are two reasons for this.  First, by running the pump, the pond water is actually being made colder to the fish who usually hibernate at the bottom of the pond where the warmer thermal layers are.  By circulating the water, the colder water near the surface is mixed with the relatively warmer water at the bottom, thus making it colder for the fish.  

The other reason we recommend stopping the pump / filter is because it is difficult to do maintenance on the filter in the cold weather, so most people neglect cleaning the filter.  The filter will ultimately clog and put excess strain on the pump.  Or, in the case of external pressurized biological filters, an unexpected power failure can cause the pump to stop and the filter to freeze and crack because it is full of water.

There also seems to be a fair amount of confusion about what to do with the plants in the pond during the winter.  Again, by following several simple steps, pond owners can prepare their plants for optimal recovery in the spring.  

Some plants, however, do not winter over and must be thrown out.  These include any of the floating plants like water hyacinths, water lettuce, floating fern, and any other non-potted floating plants.  Also, even some potted plants like tropical water lilies must be disposed of and replaced in the spring.   

Most potted plants do winter over well, provided they are properly prepared.  We recommend that the pond owner take the time to trim the plants down as much as possible.  This means trimming / cutting any part of the plant that grows up above the rim of the pot.  This part of the plant will only die and decay in the pond in the winter, so it is best just to cut it off.  The roots or plant tuber, which is well insulated in the dirt, should winter over fine if kept below the frost level of the pond.  

For this reason, we also recommend lowering the pots down to the deepest part of the pond during the winter.  If the roots are exposed to extreme cold conditions, they will die and have to be replaced, so they should be at least 18" below the surface of the water.  As an added precaution, we also recommend adding a de-icer to prevent freezing of the plant roots.  

Following the steps outlined above, you can be sure that your pond will be ready for spring and all the pond life will be as healthy as possible.