wintering koi

Question
Hello,
I have a very small koi pond that I purchased at Home Depot this past spring. I live in Rhode Island. All summer long we have enjoyed 3 fish (gold/koi?)- purchased at PetCo.  One has become much larger than the others and is about 10 inches long. The others have grown, but are about 5 inches long.
What should I do with the fish for the winter? At first, I thought I would bring them into my classroom, however, I no longer have a tank large enough for them.
Do they have to burrow in mud? I would think that given the winter temperatures in Rhode Island, that the entire pond would freeze. It is about 4 feet long by 3 feet across and probably 12 -14 inches deep. I have a fliter with a little waterfall.
Should I keep the filter running to prevent total freezing and put some dirt at the bottom of the pond and just leave the fish there?
I also have a 20x40 inground pool with a mesh cover. The surface of the pool does freeze entirely; thus eliminating the dangerous chemical waste from leaves and algae the ability to escape in the cold winter months.
Thanks for your help.

Answer
It's important to know if they are goldfish or koi.  Goldfish can grow to about 14" while koi can grow to 2 or 3 feet long.  Based on their fast growth, I would guess that you have koi.  Koi really need ponds over 1000 gallons.  Your pond is 90 gallons at most based on your dimensions.  For RI, you'd want one at least 3 or even 4 feet deep so that there was plenty of swimming room under the ice.  My page on wintering ponds is at http://www.fishpondinfo.com/winter.htm

Your three koi at this size should have at least a tank or pond of a few hundred gallons.  If you can't get an aquarium that large, you might want to buy a Rubbermaid stock tank at the local farm store.  That's going to be the cheapest large container you can get.  They come in 100 gallon sizes.  

Koi (and goldfish) do not burrow into the mud during hibernation.  They may hide under leaves.  Too much dirt or debris is bad as it decreases the oxygen levels and releases toxic hydrogen sulfide and methane gases as it decomposes.

Your pond at 14 inches deep is most likely not deep enough to over winter your fish in RI.  It bothers me that they sell those shallow preformed ponds especially in cold areas.  If you want to try, put in a de-icer.  Turn off the filter.  Adding an air stone (like from a Tetra Luft pump as shown on my winterizing page) is also a good idea.  Do NOT add dirt to the pond.  If it's a warm winter, you might get lucky.  I wouldn't want to chance it.  I suggest bringing them into that stock tank in your garage or basement for the winter and planning to either build them a larger pond next spring or find a larger home elsewhere for them.  Good luck!