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  • Writer's pictureStewart Kessel

Understanding and Avoiding ’Old Tank Syndrome’

Updated: Jan 8

How to recognise and avoid old tank syndrome



We discussed ‘New Tank Syndrome’ as part of the topic of how to set up a new freshwater aquarium. It highlighted the importance of understanding water chemistry and the role it has in helping establish a suitable and stable environment for your fish and other livestock. However, comprehending and monitoring of water chemistry is just as important in the routine upkeep of your aquarium. Best exemplified when considering ‘Old Tank Syndrome’.


A condition that as the name suggests, will often occur in aquariums that have been established for long periods. It should be noted, however, that the name is a little misleading, as under certain circumstances it can also occur in less mature aquariums.

‘Old Tank Syndrome’ typically first manifests itself when newly added fish rapidly become diseased and die in what appears to be a well-established aquarium. 


Often the immediate thought is “the aquarium has been ok until now”. Followed by “the new fish must have been diseased at the shop”. Subsequent additions, maybe from another source, also suffer the same fate.

The problem, more often than not, is actually due to an underlying issue with the chemistry of the aquarium water. Despite the situation only coming to light due to the death of the new additions, the aquarium ecosystem is very likely close to crashing.

It is worth noting that ‘Old Tank Syndrome’ can even catch-out the most seasoned aquarist. Regular maintenance, including frequent water changes, also does not necessarily mean that the water chemistry is stable. As suggested the aquarium can outwardly look well established and healthy, but without regular monitoring of the water chemistry, things might not be what they seem.

Let’s consider factors that contribute to ‘Old Tank Syndrome’:

· Infrequent or too small water changes

· Excessive build-up of dirt & decaying matter in the aquarium and substrate

· Too many fish (overstocking)

· Excessive feeding

The effects on water quality as a result of the above will include; very high nitrate levels; low pH; reduced mineral content and sometimes measurable levels of ammonia and or nitrite. Additionally, dissolved oxygen levels may also be reduced, and phosphate levels elevated. All of these are invisible without measurement and will either be toxic to your fish or will distress them making them more vulnerable to disease.

So why in the above scenario was the existing livestock seemingly ok and the new additions so adversely affected? The short answer is that all the inhabitants will have been adversely affected. The change in water chemistry will have happened slowly over an extended period, enabling the existing longer-term occupants to adjust to the less than ideal conditions. However, they will be more susceptible to disease, not be as vibrant or exhibiting peak behaviour and have a much-shortened lifespan. The new inhabitants, on the other hand, will not have been able to adjust to the adverse conditions, and the shock will have sadly caused them to perish. Neither situation being ideal.

So, let’s consider how water chemistry is affected concerning the above listed contributory factors and how it changes over time to cause the problems.

We know that in a properly established aquarium, through the nitrogen cycle, fish waste in the form of ammonia is converted into nitrite and then nitrate by beneficial nitrifying bacteria. Nitrate compared to the other sources of nitrogen is less toxic to fish. But if not reduced through regular water changes, it will with time steadily increase to dangerous levels and will start to affect the wellbeing of the fish and other livestock.

Similarly, excess food waste and other decaying matter if allowed to build-up in the aquarium, in particular in the substrate, will exacerbate the situation.

An often-overlooked consequence of a high nitrate level is that it can under certain conditions inhibit the beneficial nitrifying bacteria, reducing or in extreme cases stopping their ability to convert ammonia and nitrite. This will lead to dangerous spikes in ammonia and nitrite, both extremely toxic to the aquarium inhabitants.

In a heavily planted aquarium, the plants will use the nitrogen sources as nutrients and thus help reduce the levels. However, as with any other aquarium set-up, without regular maintenance and water changes, the nitrate level will continue to increase, albeit at a reduced rate.

Nitrifying bacteria will also be present on other surfaces in the aquarium, in particular the aquarium substrate. This will also aid the conversion of toxic aquarium waste into the less harmful nitrate. However, large build-ups of detritus, uneaten food and other decaying matter such as dead plant matter will start to clog and deprive the beneficial nitrifying bacterial of oxygen. Specific substrates, such as fine sands, will also begin to compact. In these cases of low oxygen content, anaerobic bacteria will establish and cause the production of other harmful and toxic substances such as hydrogen sulphide, as well as start to reverse the nitrifying process causing the production of nitrite and ammonia.

Stock levels will also have a significant bearing on the situation. In an overstocked aquarium, the increased number of inhabitants will create more waste and hence more demands on the system. Although in reality, it is not that simple. Some inhabitants are generally messier and generate more waste than others, for example, goldfish. Another example is herbivores, plant eaters, that typically eat more frequently and excrete more often as they can only extract small quantities of the nutritional source from the products they consume. So, it is not just stock level, but also the type of fish kept.

Overfeeding of livestock is also a major contributory factor. Excess uneaten food, as discussed above, will gradually build-up and increase the amount of waste detritus in the aquarium, as well as being broken down and adding to the harmful chemicals in the aquarium water. Not only will this be in the form of the nitrogen sources described above, but the breakdown of fish food is also a source of phosphates.

Due to low metabolism, phosphates, found in fish foods, are excreted as waste. High phosphate levels do not necessarily have a direct effect on the health of your fish, but this can cause unsightly algae blooms. During hours of darkness, this can deplete dissolved oxygen levels in the aquarium water, which in turn could harm your fish. Invertebrates, such as freshwater shrimps, are also more suspectable to being harmed by high phosphate levels.

In summary, the greater the bio-load on the system and the more waste generated, so the level of nitrate and other undesirable materials will build-up in the aquarium.

However, the story does not end there, and perhaps the biggest and the least foreseen culprit of ‘old tank syndrome’ is one where the aquarium becomes a victim of its very own success. Aquarium water contains numerous minerals. These are what affect the hardness of your water. Water depending on the levels and types of these dissolved minerals is described as either soft or hard. In turn, this affects other parameters such as pH. The specific mineral content of water is therefore not only vitally important when considering the optimum conditions for your chosen fish and other livestock, but also its stability.

Carbonates and bicarbonates measured as KH, in particular, have a critical role in the buffering capacity, or pH control, of your aquarium water. They also provide a carbon nutrient source for some plants, invertebrates and notably the nitrifying bacteria responsible for converting fish waste. Therefore, in an established aquarium with little or no maintenance, the carbonate content will become depleted with time. The depletion is more critical in soft water set-ups due to their inherently low initial mineral content. Also, a common mistake is the use of reverse osmosis (RO) water without remineralisation, which will have no buffering capacity.

The consequence of depleted carbonate in the aquarium water is twofold. Firstly, it can adversely compromise how fish regulate the minerals in their body through osmoregulation. In itself, this can be deadly. Secondly, without the carbonate buffering capacity, the aquarium water pH can drop or fluctuate widely leading to metabolic acidosis in the fish. Either of these situations can cause death, or at the very least cause stress and make the fish more susceptible to disease.

The reduction in pH is the result of the actions of the bacteria in the filter as they produce hydrogen ions as part of their nitrifying process. In combination with high nitrate levels, this will be in the form of nitrous acid. Without the carbonate buffering action, the pH will drop significantly with time.

To further complicate matters, if the pH drops too low in combination with high nitrate levels, it will also inhibit the nitrifying bacteria and thereby result in spikes in ammonia and nitrite. Both, as we know, are highly toxic to fish and other livestock.

Fortunately, at a low pH, ammonia will be present in the form of the less toxic ammonium. However, this is a hidden danger when trying to rectify the situation. For example, a good-intentioned water change at this time to reduce nitrate could cause the pH to increase. As a consequence, the ammonium reverts and creates an ammonia spike.

At very low KH, the pH of the aquarium water will also fluctuate more widely and rapidly. Respiration of fish and in particular plants at night can cause a rapid reduction in the pH value (due to CO2 production). Often referred to as pH shock, this is directly related to the lack of buffering capacity in the water.

In conclusion, a well-established aquarium, with little or no maintenance can with time become dangerously unbalanced. A gradual adverse change in water parameters, with few outward signs, can mislead the aquarium hobbyist into believing all is well with their aquarium. Without testing the aquarium water, parameters such as nitrate and phosphate can undesirably increase. The carbonate content can become depleted, causing a drop in pH, and as a consequence the nitrifying bacterial may become inhibited, leading to a dangerous spike in ammonia and nitrite.

All this sound complex, but in reality, all the factors are interlinked and ‘old tank syndrome’ can easily be prevented through correct aquarium maintenance and routine monitoring of the water parameters.

Regular aquarium maintenance to remove dead and decaying detritus; cleaning of the gravel or sand substrate; frequent partial water changes, using re-mineralised water where required and proper maintenance of the filter; all will help maintain a balanced aquarium environment. Testing key water parameters such as ammonia (NH3), nitrite (NO2), nitrate (NO3), pH, and carbonate hardness (KH) will help monitor the health of your aquarium. Additional tests on phosphate (PO4) and General Hardness (GH) can also serve to help understand your aquariums parameters.

A last word of warning, rectifying an aquarium with ‘old tank syndrome’ is often significantly more difficult and time-consuming than proper maintenance, and hence confirms the old proverb’ prevention is better than cure’.

Stewart Kessel

Owner, Freshwater Aquarium Services


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