Nitrite can cause problems with the fish. It prevents the blood from carrying oxygen properly. You would notice Koi with brown tinted fins (tail fins and other fins). The common name of this problem is Brown Blood Disease.
Add sodium chloride (NaCl) at the rate of 1 teaspoon per 20 gallons.
This is a low level of salt and will not harm fish or plants. However, once it is in there, the only way to remove it is water changes. You do not have to worry about removing the small amount of salt quickly, though. If you are doing regular small water changes eventually the salt will go away.
You might do a bit of research and look at how fish farms deal with this problem. There are other sources of chlorides that will help in this situation, for example, calcium chloride can be used.
On a smaller scale (aquariums in the 10s of gallons to a couple of hundred gallons), products like Prime can lock up the nitrite until the bacteria have recovered to deal with it. I have no idea the cost of dosing your tank size with Prime. I think the better idea is to keep up the water changes.
You can buy the proper bacteria. Look for Nitrospiros species of bacteria.
The following products have the right species:
Microbe Lift's Nite Out II
Tetra Safe Start
Dr. Tim's One and Only
there may be others, but read the label.
For such a large tank (pond?) any of these products would be very expensive if you needed to dose for the whole volume. You probably only need to add to the bacteria population, though, not replace the whole amount. These products need to be refrigerated, so if you can find a source, pay for fastest shipping.
Other sources of these bacteria are other ponds that are well cycled. The problem here is that removing the bacteria from one pond to help another might cause the same problem in the pond that donates the bacteria. Perhaps taking a small amount of filter media from several ponds, so that no one pond is losing too much bacteria.