The nitrogen cycle makes it possible to keep fish in an aquarium. However, during parts of this process, or when something goes wrong, nitrate levels can spike and endanger fish. You can take several steps to control nitrate, including making water changes and adding organisms that will consume nitrate.
What Is Nitrate?
In a healthy aquarium, bacteria break down ammonia—produced by fish as a biological waste—into nitrite, then nitrate. Each change produces a less toxic substance. While ammonia and nitrite can poison fish at any detectable level, an aquarium can have a "safe" level of nitrate. Different experts specify different safe levels of nitrate for different species, generally between 20 to 40 ppm (parts per million). Ideally you want nitrate levels as low as possible, but you are unlikely to ever achieve 0 ppm.
Water changes are the most tried and true method and the best short-term method for controlling nitrate. Make water changes part of your regular aquarium maintenance. Different experts recommend different guidelines, but if you change 25 percent of the water in a freshwater aquarium every two weeks, or 10 percent of the water monthly in a saltwater aquarium with live rock, you are probably good. Of course, also perform water changes when you detect elevated nitrate levels or see signs that your fish or inverts are stressed out.
In a freshwater aquarium, plants can absorb a number of nitrogen compounds, including ammonia, nitrite and nitrate. However, plants can do this only under ideal conditions—and keeping aquarium plants happy and healthy is harder than it looks. Most aquarium plants need intense lighting, and possibly supplemental carbon dioxide in order to thrive. Plants kept under suboptimal conditions die, rot and release nitrogen compounds as they decay. So this method of controlling nitrates in the aquarium works only if you know your way around aquarium plants.
In saltwater aquariums, live rock works like plants in a freshwater aquarium. This is composed of old, dead coral skeletons that have been colonized by other organisms, including nitrate-eating bacteria. The use of live rock makes it possible for marine aquariums to go somewhat longer between water changes than their freshwater counterparts—though controversy exists on this point.