New tank syndrome occurs when the filter in an aquarium fails to provide adequate water quality. High levels of ammonia and nitrite stress fish, making sickness and death much more likely.
What we call ‘new tank syndrome’ is best described as a series of fish illnesses or deaths attributable to poor water quality. Bacterial infections such as finrot and mouth ‘fungus’ are very common, as are dropsy and fungal infections of the fins and body. Even if they are not actually sick, most fish will respond to poor water quality in some way, often becoming disinterested in food or swimming nervously. Gasping at the surface of the tank is common, too.
Although new tank syndrome occurs when a biological filter is not yet able to support the population of fish in an aquarium, it isn’t exclusively limited to new aquaria. Old tanks can suffer from new tank syndrome too, for example if too many new fish have been added at once. So simply because an aquarium has been up and running for a few months or years doesn’t mean you can rule out new tank syndrome as a source of problems.
New tank syndrome is fundamentally a case of poisoning. Ammonia and to a slightly lesser degree nitrite are poisons. Fish absorb them through their gills and skin, and in doing so, poison themselves. A very few fish have evolved to tolerate high ammonia concentrations because of the environments where they live, for example tide pool-dwelling fish like mudskippers. But most aquarium fish will be stressed by ammonia levels as low as 0.5 mg/l (0.5 ppm). Nitrite is marginally less toxic, but again, even concentrations as low as 1.0 mg/l (1 ppm) can cause serious harm, even death.
pH affects the toxicity of ammonia significantly. The chemistry is a bit complicated, but in short, in acidic conditions the ammonia takes a form that is relatively less toxic compared to the form it takes in basic (alkaline) conditions. An overlooked aspect of this is how photosynthesis affects pH. During the daylight hours when plants are using light to make their food, they take in carbon dioxide from the water and this causes the pH of the aquarium to rise. On the other hand, plants remove ammonia from the water directly, using it as a mineral nutrient, so determining the precise role plants may play is difficult to predict.
The new tank cycling process
If the tank is matured using a fishless cycling method, then new tank syndrome shouldn’t be a problem. The usual approach here is to use household ammonia on a daily basis to raise the concentration to about 5 mg/l (5 ppm) and then to check the ammonia level a few hours later. For the first week or two it will rise, but by about the third or fourth week it should have dropped down to zero. Nitrite level rises and falls a week or so later. All else being equal, the entire process should be finished within six weeks, or a bit longer for coldwater tanks. At that point ammonia will not need to be added any longer, and the first batch of fish can be added to the aquarium.
However, if an aquarium isn’t cycled with fish, new tank syndrome becomes a major issue. The problem here is that while the fish are producing the ammonia the filter needs to mature, not all that ammonia will be getting used up. The ammonia left over poisons the fish, causing stress and making them more vulnerable to infections. The same holds true for nitrite; ordinarily this is produced by filter bacteria as part of the biological filtration process, but immature filters won’t have enough bacteria to use up this nitrite at the same rate it is produced. So again, nitrite levels in the water rise, stressing the fish.
New tank syndrome is handled by keeping ammonia and nitrite levels as low as possible. Because ammonia test kits detect not only toxic ammonia but also harmless ammonia-containing compounds produced by water conditioners, ammonia test kits results can sometimes be misleading.
Nitrite test kits are perhaps the easiest way to monitor the cycling process. For the first week or so nitrite levels may be zero because there aren’t many of the bacteria that turn ammonia into nitrite, but the nitrite level should quickly rise to a peak within the second or third week. Use the nitrite kit every day or two, making sure nitrite is below 0.5 mg/l (0.5 ppm) and ideally as close to zero as possible.
In practice, doing 20-25% water changes every day or two should keep both ammonia and nitrite levels below tolerable limits, provided (and this is important) the number of fish added to the tank is small and they are being fed very little. For a 20-gallon aquarium, a group of six young zebra danios would be the maximum that could be safely held in the tank during its cycling phase. They should be fed only every 2-3 days, and not at all if they are exhibiting signs of stress.
Salt has a mildly detoxifying effect on nitrite, so can be useful during the cycling process at doses up to 0.5 oz/US gallon. Higher concentrations may be used where salt-tolerant fish such as guppies and mollies are being kept, but otherwise salt shouldn’t be leaned on as a cure-all; it’s not.
Plants, shrimps and snails
Fast-growing plants lower ammonia concentration by removing it from the water as a mineral nutrient they need to grow. Plants also carry bacteria with them, and thereby help to ‘seed’ the filter, speeding up the cycling process. However, in new tanks it may take time for plants to become established, and unhappy plants, such as those languishing under poor lighting, won’t be much use at all.
Nonetheless, one way to minimize new tank syndrome is to spend the first week or two stocking the tank with plants before adding any fish. Many aquarists have found that adding snails or shrimps also helps. These produce ammonia, but less than fish, easing the biological filter along without ruining water quality. After two or three weeks of this, the cycling process will be well underway, and a few fish can be added at less risk than if they were added right from the start.