Is this cause

nitrate is the end of the nittrogen cycle in your pond unless you have plants or algae growing in there.

you didn't say how big this pond is... if it were a tank and you had that nitrate level I would recomend a water change or two or three, because that is maybe not high enough to cause your fish to get sick but it may be stressing them..

with out knowing more about the details of this pond I don't know what to recomend.... is it indoors or out side? how big?
what are you using to circulate the water?

You might need to do a water change twice a week to get the nitrates to stay down. Even then it might not be enough. First though, are you having and problems? Any fish dying? Remember, if it ain't broke, don't fix it. (even though water changes are good )

Is this an indoor or outdoor pond? Where are you located? If your have koi/goldfish hybernating.... do not do any water changes!!!! Keeping ponds is very different than aquariums.