Nitrates are the last chemical produced in the nitrification cycle. They are relatively not toxic compared to ammonia and nitrites but can kill fish at levels over 300 ppm. In most ponds, they are consumed by the algae in the pond well before they kill the fish. At levels below 80 ppm nitrates are not a problem. Levels above 80 ppm should not be ignored, as they can cause redness in the fins and cause fish to become very lethargic. High nitrates can also cause stress to the fish, which in turn can prevent soars from healing even with the use of antibiotics. High nitrates can also stunt the growth of fish.
What can cause high nitrates?
If the pond is entirely void of all plants, including algae, there is nothing to consume the nitrates. This would allow the nitrate level to go up. Overmedicating a pond with potassium permanganate or formalin can kill the algae in the pond. Some algae control treatments will wipe a pond clean of algae. Very little to no sunlight on the pond may inhibit algae growth also.
How do I lower high nitrates?
First, perform water changes until you get the nitrate level in the acceptable range. Next, consider slowing down on feeding the fish for a while. The more food you give the fish, the more nitrates they will give you back. Then go about solving the problem long term. You could place water plants such as Hyacinth or Water Lettuce in the pond to help consume the nitrates.
For those who would prefer no plants in their ponds, of which I am one, you will need to cultivate a healthy layer of green algae on the sides of the pond. If the pond, at present, has a layer of brown algae, you might want to brush it from the sides to make way for the green algae. Make sure the pond bottom is clean of any debris that might prevent algae from attaching to the pond bottom. Finally, make sure the pond gets some sunlight during the day. If you cultivate a nice healthy inch-thick layer of green algae on all the surfaces of your pond, your nitrate levels should be well under control.