Many novice aquarists become disillusioned at the frequently experienced high death rates of their aquatic pets after setting up a new aquarium. Statistically, as much as 60% of the fish sold to novice aquarists for a newly setup aquarium will die within the first 30 days. Commonly known as "New Tank Syndrome" these fish are poisoned by high levels of ammonia that are produced from bioconversion or decomposition of fish wastes, dead animals and plants, uneaten food, bacteria and other organic matter. Additional ammonia is excreted directly into the water by the fish themselves.
In aquarium systems, ammonia is oxidised by nitrifying bacteria to nitrite, which is also toxic, and then to nitrate, which is not as toxic, in a process generally referred to as the biological cycle or nitrification. Therefore, establishing and maintaining an adequate healthy population of nitrifying bacteria that is capable of reducing the ammonia levels is critical to success when fish are first introduced into a new aquarium. If the appropriate bacteria are not present or if the correct physicochemical conditions are not maintained, nitrification will not function adequately. This may lead to an accumulation of toxic ammonia or nitrite. When this happens the fish will be stressed, and may even die.
Ammonia is toxic at low concentrations and must be removed at a rate equal to production to maintain water quality at a level adequate to prevent exposure to the fish. The short-term (48-96 hours) LC50 for rainbowfishes has been reported at concentrations of 0.57~0.75 mg/L. All juvenile (34~40 mm) rainbowfishes (Melanotaenia splendida) exposed to this level died. Nitrite toxicity can also be a problem in newly set-up aquaria. If any fish are dying in your aquarium 12 to 15 days after it was setup, the chances are that they are being killed by high nitrite levels and not high ammonia levels. The tolerance to nitrite exposure varies greatly among fish species. The short-term (48-96 hours) LC50 values varies from 0.5-1.5 mg/L for many fish species. However, there is little information available about the susceptibility of rainbowfishes to rising nitrite levels. Waterchanges of 25% every 2 or 3 days for the first three weeks or so should reduce high nitrite levels.
It is generally recommended that long-term ammonia and nitrite levels in an aquarium should always be zero when using common test kits available from pet stores. Presence of either is an indication that something in the system is out of balance and should alert you to start corrective action. If low levels of these elements persist they will cause chronic problems for your fish and increase their susceptibility to other disease conditions.
When an aquarium system is first setup or restarted after cleaning, time must be allowed for the nitrifying bacteria to become established. Bacteria are present everywhere and reproduce at astonishing rates. Many of these bacteria live in biofilms that are located on all surfaces and in particular in the biofilter, but they are also present within the water column. During the time it takes for the bacteria to become established ammonia levels can get very high. This is a critical time in aquarium systems, because ammonia levels may increase faster than their removal. Several days after reaching their peak level, ammonia-consuming bacteria convert them to nitrite. As ammonia levels begin to fall, nitrite levels begin to rise and continue to rise for some days until nitrite-consuming bacteria levels are sufficient to consume the nitrite and convert it into the relatively harmless nitrate. This cycle is generally referred to as the nitrogen cycle.
It is not critical that you understand the intricate metabolic process that naturally occurring bacteria use to create and maintain this cycle. However, a reasonable knowledge of the major steps of the cycle and what effect, it has on water quality is required. The experienced aquarist however, realises the importance of establishing the nitrogen cycle quickly and with minimal stress on the aquarium's inhabitants.
The nitrogen cycle in aquarium systems is shown above. The two processes in the nitrogen cycle that is of major importance in aquarium systems are nitrification (shown in green) and denitrification (shown in red). Ammonia is oxidised to nitrite and then to nitrate through a series of biochemical reactions called nitrification. Denitrification is primarily a reduction of nitrate to nitrogen gas by anaerobic bacteria.
It is often suggested that the nitrogen cycle can be started artificially by adding ammonia, such as ammonium chloride, into the aquarium water, thereby allowing establishment of the nitrifying bacteria prior to stocking with fish. It is thought that by adding the ammonia, it could enhance the bacterial reproduction, which might shorten the time needed for these bacteria to become established. However, the effect of exogenous addition of ammonium chloride can extend the cycling time dramatically; in some cases, as much as 50~60 days.
It may be advantageous to pre-activate the cycle. Pre-activation is accomplished by seeding the aquarium with a 'starter' bacterial population. Seeding of nitrifying bacteria has a positive effect on the nitrification process by improving performance and stability. Nitrifying bacteria are usually abundant in the gravel substrate or filters of an established aquarium; so many hobbyists use a small amount of this as a bacterial seed. Experience has shown that seeding filters with filter media from established filters can significantly reduce new system start-up times. Previous studies have shown that seeding freshwater systems with 3% of wet filter media from an established filter decreased the start-up time of a newly setup aquarium by 48%. The addition of 10% wet filter media from an established system can reduce the start-up time to around 4~7 days compared to 20~25 days for ammonia removal and a similar time scale for nitrite removal. However, seeding with a wet media from an established aquarium has a major disadvantage because it might cause transfer of diseases even after the nitrogen cycle has become established, as infectious bacteria might survive such conditions.
There are many commercial products on the market today that claim their usefulness for this purpose. However, commercial bacteria cultures can produce variable results. Care must be taken to understand the nature of the products and often what is not stated on the labels or perhaps not even understood by uninformed manufacturers. Some commercial products contain bacterial soup while others have an ammonia-based composition. Some bacterial products are somewhat over-rated and in fact, some claims are erroneous.
As an alternative to preactivation, a staggered stocking regime can be used. Once the aquarium water is stable, you can increase the animal load to the desired level. The aquarium is stocked with a small number of fish. The bacteria will grow on the wastes from the animals and plants, and equilibrium will be established between the quantity of the wastes produced by the fishes, invertebrates and plants and the bacterial population. An increase in wastes will result in an increase in bacteria, but the response is not instantaneous. The time required is a function of the conditions under which the bacteria are growing and the nature of the bacteria themselves. When the biofilter bacteria populations grow to meet this ammonia load, additional fish can be added. However, when this technique is used, water changes may be required. Change about 15% of the water every 3 days for the first 15 days and you should have no problems. To reduce the maximum amount of ammonia make sure you do not overfeed the tank.
To date, it has been reported that nitrification in aquarium biofiltration is performed mainly by organisms of the Nitrosomonas and Nitrosospira genera. However, recent studies have revealed the widespread existence of unique ammonia-oxidising archaea (bacteria-like organisms), belonging to the Archea domain. New research is showing that archaea capable of ammonia oxidation are ubiquitous in marine and fresh water environments. The phylogenetic diversity and species richness of ammonia-oxidising archaea and bacteria were examined in aquarium biofiltration systems. The results showed that species richness of ammonia-oxidising archaea is greater than those of ammonia-oxidising bacteria. However, the relevance of ammonia-oxidising archaea in aquarium systems remains unknown. Various heterotrophic bacteria, algae and fungi have also been reported to have nitrifying activity, although at a slower rate than nitrifying bacteria. Therefore, the process of nitrification is much more complex than indicated in most aquarium textbooks.
It is best not to add any fish to a newly established aquarium until the biological cycle has been established and ammonia and nitrite levels are zero. This requires variable lengths of time, but it should generally be safe to start introducing fish after 21~28 days, but use test kits to monitor the process. All hobbyists should have an ammonia and nitrite test kit and know how to use them. You must monitor water quality and be able to correct any problems when and if they occur, as there is not much forgiveness if something goes wrong.
The stocking density, that is the volume of water available to a single fish, can be a significant health factor. Unfortunately, people who commence keeping aquaria are usually too anxious to have as many fish as possible, and most of the problems which overtake their endeavour arise from such over-stocking. Large numbers of fish confined in the relatively small space of the aquarium can often lead to impaired growth and health conditions, and before long it ends in death. This is due to reduced oxygen levels and increased toxic substances such as ammonia and nitrite.
Although there are numerous mathematical formulas to calculate the fish holding capacities of aquariums, they are essentially without merit. The maintenance of water quality is the best method to determine the stocking density. Start with a very small number of fish and check the water quality using test kits for ammonia, nitrite, and pH. More fish can be added gradually, building up over the months as your aquarium matures, as long as the water quality is not diminished. Ultimately, your own observations and test results should help you determine what is a safe number of fish for your particular situation. Most important of all, do not add too many fish to the aquarium at the same time.
© Copyright Adrian R. Tappin
Updated February, 2009.
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