Mastering Freshwater Aquarium Ecosystems
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Nitrogen Cycle (Cont.)

Nitrite levels will peak around three weeks after adding fish to a new aquarium setup. It is extremely important to note that nitrite is the number one cause of fish mortality in a new freshwater aquarium setup (New Tank Syndrome), not ammonia.

Nitrate (NO3)

Nitrate is final chemical in the nitrogen cycle. In most freshwater aquariums, nitrate builds up over time. The rate of accumulation of nitrate is directly related to fish waste, decay of aquatic plant matter, and if there are any natural nitrate reduction (NNR) strategies employed within the aquarium ecosystem. Aquariums with a heavy fish load, that are fed often with high protein food, can accumulate nitrate very quickly, often well over 100 ppm in a month.

In natural bodies of water, nitrate is rarely detectable (<1 ppm) with an aquarium test kit.

While nitrate is not as toxic as ammonia and nitrite it can have an adverse affect on the aquarium ecosystem. Fish that are exposed to very high nitrate levels (> 400 ppm) for extended period of time have been know to develop a hole in the head (HITH) condition where the skin around the head recedes. HITH condition is commonly seen in Oscars (Astronotus ocellatus), and other large Central and South American cichlids. Excessive nitrate can affect reproduction, fish growth rate, and the immune system's ability to protect against disease.

Keeping nitrate as close to 0 ppm is optimal. As nitrate increases it will also lower the aquarium's pH. This can be a problem for fish that are adapted to a higher pH range and in systems that have low alkalinity. In freshwater aquariums, nitrate should not be allowed to exceed 100 ppm for more than a month.  In systems that have a low alkalinity level, 100 ppm of nitrate can easily lower the pH below 6.0.  Low pH has been linked to the lower oxygen carrying capacity of the fish’s blood by as much as 60%.  A low pH and high nitrate level is double trouble for the fish’s ability to take up oxygen.

Natural Nitrate Reduction (NNR)

Nitrate typically builds up in the aquarium over time, but it can be slowed down and even completely kept from rising. NNR can be accomplished by implementing nitrate reducing elements into your aquarium ecosystem, and keeping fish populations (biomass) in check.

Fast growing live plants under bright light will take up nutrients from the decomposition of fish waste, uneaten food, and plant matter. The brighter the light, the faster the plants will take up nitrate. An aquarium positioned in the home so that it will receive a couple of hours of natural sunlight can help reduce nitrate, as well as help bring out the natural colors of fish. Most species of aquatic plants prefer ammonium over nitrate. As fish release ammonia, most of it will be converted to ammonium and thus available for plants as a nutrient. Plants in a brightly lit aquarium are one of the best ways to control nitrate accumulation in the freshwater system.

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