The Nitrogen Cycle

Nitrite

The next product in the nitrogen cycle is also the most toxic as it can cause immediate blood changes that blocks the transport of oxygen through the body (at levels as low as 1.6 mg/L). Also exist in two forms, but in more acidic water the amount of the more toxic form (nitrous acid) is higher, in contrast to that of ammonia (lower pH is related with the less toxic form). In softer water, the same is true.

Compared to ammonia to nitrite conversion (through oxidation or reduction), nitrite to nitrate conversion takes a longer time as the bacteria (nitrospira) divides more slowly (24 hours to double the population). Thus commonly, it takes 3 weeks and longer to establish enough population of the nitrospira to convert nitrite to the least critical nitrate. At all times, the nitrite level should be maintained below 0.1 ppm or 1 mg/L.

Nitrate, the final by-product of nitrification, although less toxic than the two earlier by-products should be maintained below 50 ppm as it can stress the fish and lessen the resistance to diseases. Despite the popularity of using aquatic plants to remove nitrate from the tank, a more recent study provided evidence that most aquatic plants preferentially use ammonia than nitrate. The only means to lower nitrate levels is through regular partial water changes.

In all these processes, it is important to note that oxygen is a requirement. Both stages are also light-sensitive (more so with nitrospira).

To be cont'd...