• Symptoms (caution: fish do not necessarily show all the symptoms at once!): fish may gasp, be listless, feed little or not at all, and have fearful behaviour and rest on gravel with clamped fins.
  • Other possible symptoms: black smudges on skin, red-veined fins, loss of color, bright red gills with possible bleeding and breathing difficulties (accelerated gill movement).
  • Possible Causes: the most common cause is poisoning due to the presence of ammonia and/or nitrite, both of which are highly toxic to fish.

Ammonia poisoning is the leading cause of mortality in aquarium fish. Typically, there is a sharp rise in ammonia just after an aquarium is first set up. However ammonia build-up is also possible in an older aquarium if the aquarium is too small and/or overcrowded or poorly maintained (e.g. no water changes), after a prolonged power failure (during which the bacteria in the filter died), or due to a filter failure or inadequate filtration.

Has your aquarium been set up recently?

As soon as the fish are added to the aquarium, they begin to produce a large amount of organic waste, especially highly toxic ammonia, through feces, urine, breathing (plus possible waste from uneaten food or rotten leaves…). The role of the tank filter, which houses billions of purifying bacteria, is to eliminate this waste through a series of transformations:

  1. First, the filter bacteria transform the very toxic ammonia into another, slightly less toxic substance, nitrite;
  2. Then they transform the nitrite (with an “i”) into nitrate (with an “a”), which is not very toxic except in high doses (note: to keep the nitrate level reasonably low, you simply change the water partially every week). This series of changes is carried out constantly and “in a loop” in the aquarium: this is called the nitrogen cycle.

Unfortunately, a new tank does not harbor enough beneficial bacteria to do this “cleaning job” properly, as it takes an average of 4 weeks for the bacteria to grow in sufficient numbers in the filter. Therefore, during the first few weeks, ammonia and nitrite build up, poisoning or even killing fish: this is called new tank syndrome.
Warning sign

Setting up the nitrogen cycle: precautions and solutions

  • Always start with a limited number of fish so that the filter can gradually adapt to the waste production. Then increase the tank population very gradually to give the filter enough time to adapt.
  • Do frequent partial water changes during the nitrogen cycle to reduce the ammonia and nitrite concentration in the aquarium. Inexpensive water tests are available in pet stores: easy to use and instant, they will allow you to find out if your aquarium has a high level of ammonia or nitrite and decide when a water change is required. Use a water conditioner (dechlorinator) when making water changes.
  • Feed your fish as little as possible if you detect ammonia or nitrite, in order to reduce the amount of waste produced in the aquarium.
  • Oxygenate the aquarium well (with an air filter if necessary) because some of the purifying bacteria are aerobic, which means that they need oxygen to live and multiply.


After a few weeks, the filter bacteria will have established themselves in sufficient numbers to effectively treat the waste: the nitrogen cycle is then complete and the aquarium healthy (0 ammonia and 0 nitrite).

Sudden ammonia or nitrite spike in an established tank? Possible causes:

  • Sudden large increase in stock
  • Filter failure or long power cut
  • Some fish treatments may affect the filter bacteria (read the medication instructions carefully)
  • Oxygen level is too low (some of the filter bacteria need oxygen to live)
  • Inadequate tank maintenance (dirty aquarium)
  • Cleaning the filter mass of the filter with tap water (chlorine has destroyed all or part of the purifying bacteria)
  • Changing the entire filter media at once

Precautions and suggestions:

  • Keep a reasonable number of fish for the volume of your tank. Avoid mini-aquariums (or goldfish bowls) at all costs. Do not add too many fish at once.
  • Backup filter: having even a cheap backup filter on hand (e.g. a basic sponge filter or box filter) can save the life of your fish (main filter failure for example). This filter may also be used in a quarantine tank.
  • Battery powered air filter or aquarium oxygen tablets: in the event of a power cut, goldfish are not as affected by the drop in temperature as tropical fish (for the latter, placing a blanket around the tank can help limit the drop in temperature), but if the power cut  lasts more than a couple of hours, oxygen levels can drop very low, affecting both fish and beneficial bacteria. A battery powered air filter or – more economical – a few oxygen tablets (available from any good pet store) can help. Note that the bacteria bed may take time to return to normal levels afterwards: the water should be tested in the following days to make sure that everything is OK.
  • Treatments: if you are adding a treatment in your main tank, read the instructions carefully to check whether or not this treatment may affect the beneficial bacteria. If this is the case, a quarantine tank is a good option so that you don’t have to restart all or part of the nitrogen cycle.
  • The air filter (aerator) increases gas exchanges and therefore allows better oxygenation of the water. It is essential in an aquarium that does not have plants and in the absence of current on the surface.
  • Aquarium maintenance: The aquarium requires regular maintenance, such as checking the equipment, changing the water, etc. When the filter flow rate drops significantly, the filter media (sponge for example) should be cleaned with water from the tank, not with tap water because the filter media contains essential beneficial bacteria that are very sensitive to chlorine. If the aquarium filter is equipped with only one sponge (internal power filter for example), care must be taken before changing the sponge to avoid sacrificing the entire bacterial colony at once: simply cut the sponge in half. Only one half will be changed at a time and the second half two weeks later. The old block will then have enough time to colonize the new one and enough bacteria will be preserved! The aquarium filter must run 24 hours a day, 7 days a week.