I made a thread awhile back called truth about nitrates and do to some technical problem it got deleted on accident.
Karl has stated in a sticky thread that high nitrates can lead to ulcers. I do feel nitrates at 40 ppm or less is important to long term health. I had about 80-120 ppm before and had ulcer problems 2 winters in a row. I've since improved filtration and have 20ppm now and my fish are doing well and super active even at lower water temps. we often check for ammonia and nitrites but nitrates is important as well.
This is karls advice.
Nitrate: "Some is always present". A trace is fine. More than that: do a water change. Nitrate is called the silent killer. It is usually not monitored very well by the average koi keeper. If the levels slowly become high due to lack of water changes it will cause ulcer disease and death. Symptoms are often called "dropsy". The most common reason for Nitrates to rise is lack of water changes.