The Nitrogen Cycle
by Andrea Watts

Introduction

One of the most common and freely available elements on Earth is nitrogen. The air that we breathe comprises of about 78% nitrogen and it appears in different forms throughout life’s various biological cycles. Being an enclosed system, an aquarium requires careful monitoring to ensure that harmful forms of nitrogen do not impact negatively on it’s occupants. An understanding of how the Nitrogen Cycle works is an essential component to successful fish keeping. The chemical reactions which take place in nature are complex and an aquarium is only a reconstruction of a part of the natural environment. The scale may be smaller, but essentially the fundamentals remain the same.

Hopefully the following article will explain the processes involved in cycling nitrogen.

Ammonia

The first stage in the Nitrogen Cycle begins as organic wastes start to decompose. These wastes originate from uneaten food, undetected fish corpses and excreta from the tank’s inhabitants. The subsequent breakdown of the molecules in these wastes leads to the production of ammonia (if the pH of the water is neutral to alkaline) or ammonium ions (if the pH of the water is acidic). The bacteria responsible for the generation of ammonia are from the genera Bacillus, Bacterium or Micrococcus. The level of ammonia steadily increases as the tank ages, through the process of biochmical oxidation or ammonification. A peak of about 11mg/L is reached around day 7.

At this point in time, the bacteria responsible for the conversion of ammonia are numerous enough to reduce its concentration. The ammonia levels then drop abruptly and by day 11, its presence is virtually nil. If high levels are still detected beyond this time, or become apparent in an established tank several months later, a shortage of bacteria is indicated. There are two case scenarios to consider: increased ammonia is usually attributable to an excess of uneaten foood or undiscovered corpses. A mass destruction of bacteria is normally a direct result of over zealous water changes, the use of medications, replacement or the washing of filter media in tap water or a substrate change.

A little scientific notation for the chemistry buffs

As previously mentioned, the form of ammonia found in an aquarium is pH dependent. Free molecular ammonia (NH3), is a dissolved gas. It generally forms in tanks with a pH of 7 or greater (ie. neutral to alkaline environments). This is the form that keepers of Malawi and Tanganyikan type biotopes need to be most aware of. The toxity of ammonia is increased further in tanks with warmer water. Under these conditions, ammonia is extremely toxic to fish and becomes fatal in a very short period of time.

Biotopes that are acidic (ie. have a pH less than 7), ammonia combines with a water moleclue to form ammonium hydroxide (NH4OH). In this form, ammonia is approximately 100 times less dangerous to fish.

Therefore, African cichlids are at a higher risk than their South American counterparts due to the highly mineralised and alkaline environment. The presence of free ammonia is often indicated by a darkening of fish colour, skin damage and gill irritation (observed through fish gasping for air at the surface). It becomes fatal above a concentration of 0.02mg/L.

How do I rectify this situation?

Partial water changes will reduce the concentration of ammonia in the tank, whilst the addition of new bacteria will help re-populate the colony. Products such as Cycle, Environ-8, Nitrovec etc. are useful. An ammonia test kit, either colorimetric (colour scale) or electronic, is an essential addition to the home.

Nitrite

The second stage of the Nitrogen Cycle involves the activities of bacteria from the Nitrosomonas or Nitrosococcus genera. Nitrites (NO2), play an integral part in the decomposition cycle of nitrogenous waste in the aquarium. They are produced when the afore-mentioned bacteria decompose the ammoniacal derivatives (NH3 and NH4). From around day 6 to day 17, they convert the ammonia into nitrites. At around day 24, the nitrites begin to decline until such time that they are undetectable.

Although not quite as toxic as NH3 and NH4, nitrites are harmful to fish regardless of the pH. Therefore, fish should not be introduced into an aquarium that shows its presence. Nitrous bacteria are practically non-existent in new aquaria. To start the process, you can introduce micro-organisms that are readily sold at your LFS (Cycle, Environ-8, Nitrovec and so on). If you already have an established tank, you can call upon that reservoir of bacteria. Simply through the transfer of some filter material, aquarium decor or a handful of gravel, you can add some of the nitrous bacteria to your new tank. The transfer of water does not have the same beneficial effect as only a small amount of bacteria is suspended in the water column.

Nitrite levels should be checked regularly through the use of a colorimetric or electronic test kit. The slightest trace of nitrite can prove fatal and even when the levels register as zero, care should be taken to introduce fish gradually. Established aquariums are stable and all efforts to maintain the equilibrium must be observed. This includes not harming the bacteria through over zealous cleaning of the filter material. Do not use detergents or chlorinated water in the cleaning process and water changes should not involve replacing too large a volume of water. Fortnightly replacement of around 25% water is far more preferable to larger changes at less frequent inetrvals. Note that some medications (particularly antibiotics) can also destroy valuable bacteria. Isolating and medicating fish in a smaller tank is a better method of treatment.

Unless action is taken immediately, the fish’s health will suffer drastically. The nitrite level should not exceed 0.01mg/L. Nitrites combine with haemoglobin (the oxygen carrying component in blood) and form methaemoglobin, thus preventing the transport of oxygen. The fish turns dark brown in colour and death ensues quite quickly. The presence of nitrites in the water is indicative of poor water quality, a situation that will aggrevate quite a number of diseases.

How do I rectify this situation?

A lack of balance can arise when a weakness in the bacteria or a rise in nitrates (through oxygen depletion) occurs. This situation can be rectified through a water change and the introduction of supplementary bacteria.

Nitrate

From about the 5th day after filling an aquarium, nitrates begin to form, although it is only after about day 25 that the different populations that bring about the Nitrogen Cycle are in large enough numbers for all stages of the cycle to take place similtaneously. The intermediate toxic compounds, ammonia and nitrite, should be no longer detectable. The bacteria responsible for this activity are from the genus Nictrobacter.

This is the stage at which fish can be successfully added to the aquarium. It should be noted, however, that even though plants assimilate the nitrates originating from the breakdown of organic wastes, they are rarely in sufficient numbers to absorb all that is produced. This fact becomes more apparent in many African type biotopes that are devoid of plants altogether. Over-populated, closed environments become subject to a steady build up of nitrates. An unattended tank can emass concentrations in excess of 1g/L over a 1 year interval (as opposed to a natural environment where the concentration rarely exceeds 5mg/L due to permanently changing water).

Excessive concentrations of nitrates are often termed ’silent killers’. This is because in small doses, these compounds are not deemd toxic. An accumulation that leads to concentrations in excess of 80-100mg/L will prove fatal to your fish. Only regular water changes will reduce their concentration. Certain species are more susceptible to nitrate poisoning than others. Amounts in excess of 50mg/L are often signalled by the explosive growth of algae. In a planted tank, preferred water plants are compromised by the algae that feed on the nitrogenous compounds. Affected fish will display signs of stress, hence making them more vulnerable to opportunistic diseases. Poor water quality can provoke such conditions as ‘pop-eye’ and fin rot. Care should be taken to keep nitrates under 15mg/L.

Additional latent risks can lead to dramatic consequences. For example, if the environment is oxygen depleted due to poor maintainance or a sudden rise in temperature, favourable conditions for the bacteria from the genus Bacillus develop. This, in turn, reverses the Nitrogen Cycle through denitrification hence producing ammonia and nitrite. If this is allowed to occur, the entire fish population can be decimated within a few hours.

How do I rectify this situation?

Commercial denitrifiers work on the principle of external denitrification. The nitrate charged water circulates through an anaerobic rich environment which breaks down the nitrates into molecular nitrogen. The provision of ‘food’ (for the bacterial colonies) is the only constraint and manufacturers have developed specially adapted foods in a number of different forms.

However, there is no substitute for regular water changes and testing for nitrate concentrations.

Summary

Ammonia is extremely toxic to fish. It causes swelling to the gills, leading to asphyxia and disruptive osmoregulatory functions. Nitrites become fatal by acting on the haemoglobin in the blood. Nitrates, however, are only toxic in high concentrations (between 50 and 300mg/L depending on the species).

Therefore, only when ammonia and nitrites have become undetectable, should fish be introduced into a newly installed tank. This usually occurs around week 3 - 4. Various test kits are available to monitor the levels of ammonia, nitrites and nitrates - it is vital to use them regularly in a newly set-up tank. Those of us who are impatient can help to speed up the natural processes by introducing gravel or filter media from an established aquarium.