Nitrates are a product of the nitrogen cycle --- a cycle that occurs in the water of aquariums and other bodies of water. When fish excrete, waste is converted to ammonia which then becomes nitrites. Eventually these nitrites turn into nitrates which then become nitrogren. Nitrite levels are a particular concern for aquarium owners because nitrite is very harmful to fish--- when left untreated, high nitrate levels in an aquarium can result kill all of your fish.
Determine the levels of nitrate in your aquarium water with an ammonia and nitrite test kit. The ideal level of nitrites in the tank is 0 ppm. You should strive to keep the nitrite levels as low as possible.
Keep a small number of fish in the tank for the first three months. When you start up a new aquarium the nitrite levels are at their highest during the first three months. The more fish you add, the more waste they will create and in turn the more nitrites you will have. Keep the fish population low until you have established a very low nitrite level in the tank.
Avoid overfeeding the fish, which can create more ammonia in the tank. When you feed your fish too much, the leftover food decomposes on the bottom of the tank and turns into ammonia, and then nitrites. Feeding your fish too much is one of the causes of high nitrite levels in tanks.
Complete a large water change. If the nitrite levels in your tank have reached a point that becomes dangerous for your fish, remove 50 per cent of the water and replace it with fresh water.
Ensure that your filtration system is cycling the water properly and that the aquarium's filter is the appropriate size for the tank. The filter removes ammonia and nitrites from the tank and when it is working properly, you should never have a problem with high nitrite levels.
Add a water conditioner that will detoxify the ammonia and nitrites in the water. There are chemicals available that will lower the levels of nitrates in the water.