Nitrates are a natural by-product of the bacterial "reduction" or removal of Ammonia and Nitrite in the natural pond's ecosystem. Nitrates are an under-estimated fish killer. When fish are sick, and the history contains some information to suggest the pond has been set up for a while, you can bet Nitrate levels are part of the problem. This is especially true for Goldfish and "flipover" disease. Once dismissed as a "non" threat to Koi and Goldfish, exceptional informatioin exists to suggest this is not true. Scientists initially evaluating Nitrates as a toxin did not test their subjects long enough. Nitrate accumulations cause dilation of the veins in the fins and other health problems. Never let your nitrate levels exceed 100 ppm or illness and vulnerability to disease will be the result!
Pros: Nitrate test kit is easy to run and interpret.