Nitrite suffocates your fish.  The nitrite molecule binds with fish's hemoglobin making it tough to carry oxygen through the fish's body.  Common signs of nitrite poisoning are the fish hanging out by a waterfall (or other oxygen-rich area in the pond), gasping at the surface and/or red streaks in the fins.

You want a zero level at all times.  Unfortunately, nitrite is not as easily reduced as ammonia is through a water change.  Initially, the level will drop --- but within a few hours, it will have bounced back to where it was before the water change.  The safest way to weather your fish through a nitrite spike is with the addition of pond salt to the pond.  The chloride ion in the salt bonds with the nitrite molecule relieving the symptoms in the fish.  Salt level should be maintained at 0.10% during any nitrite influence.

Once again, feeding should be ceased or minimized depending on the level.  It usually takes an additional 2-3 weeks for a new biofilter to convert nitrite into nitrate.

Some of the more common REASONS for nitrite spikes in established ponds are:

1 - build-up of crud

2 - low KH

3 - low oxygen

4 - overfeeding

5 - cold-water feeding

6 - overstocking

7 - inadequate filtration

8 - the filter was cleaned TOO well


1 - get rid of the crud

2 - raise the KH with baking soda

3 - increase aeration

4 - cut back on food

5 - nitrite should be monitored very closely coming into and
     going out of winter --- cease feeding until nitrite reads 0

6 - get rid of some fish or build a bigger pond

7 - get a more efficient filter

8 - never clean the entire filter in one shot --- if chlorinated water
     is used, rinse the filter components in a pail of pond water 
     before putting them back in the filter