In the aquarium hobby, what is invisible is often more important than what is visible. Not all water is the same, and the differences come down to several parameters. One of these parameters is oxygen. We generally don't think much about it, assuming that oxygen won't cause any problems... Until we find our fish desperately trying to breathe at the surface.
Let's take a look at everything you need to know about oxygen in an aquarium to ensure optimal oxygenation!
Where does the oxygen in your aquarium come from?
Plants and fish breathe the same oxygen we do. The difference is that we obtain it from the air, while fish must wait for oxygen to dissolve in the water before they can breathe it. This process of oxygen dissolving into the water is essential. The water's surface is an area of gas exchange. We call it an exchange because both the water and the air gain an element when they meet at the surface: the water absorbs oxygen, while the air receives carbon dioxide. This process is continuous: the water absorbs oxygen, which fish and plants breathe, and these fish and plants then release carbon dioxide, which is subsequently released into the air at the surface.
The larger the surface area, the more efficient this exchange becomes. This is one of the reasons why very tall aquariums are generally not recommended: their surface area can be too small to provide sufficient gas exchange, which may cause problems. Oxygen can also enter the water in a second way: through plants. Live aquarium plants function much like our terrestrial plants. In particular, they produce oxygen when exposed to light and consume oxygen when the lights are off. A heavily planted aquarium can therefore benefit from a significant amount of oxygen during the day thanks to the plants.
How much oxygen does your aquarium need?
Several factors influence the amount of oxygen your aquarium water can contain:
Water temperature – As water temperature increases, the amount of oxygen the water can hold decreases. Cold-water fish therefore naturally have access to more oxygen than tropical fish. Be careful during heat waves!
Salinity – Salinity refers to the amount of salt dissolved in the aquarium water. The saltier the water, the less oxygen it can hold. Freshwater aquariums have a salinity of 0, so this distinction mainly concerns saltwater aquariums. You can test the salinity of your aquarium using a refractometer.
Atmospheric pressure – The lower the atmospheric pressure, the less oxygen the water can hold.
Time of day – Aquarium plants produce oxygen when the lights are on and consume oxygen when the lights are off. It is therefore natural to have more oxygen during the day and less at night when the lighting is off. Oxygen requirements also depend on the fish present in the aquarium. Not all fish are equal when it comes to oxygen uptake. For example, Gouramis and Bettas can naturally absorb oxygen from the surface thanks to an organ called the "labyrinth." The amount of oxygen present in the water is therefore less critical for them! More generally, large fish and highly active fish require more oxygen.
Keep in mind:
There are many factors involved, making it difficult to provide one precise answer. Generally speaking, for a freshwater aquarium heated to 24 degrees Celsius, an oxygen level of around 8 mg/L is ideal. This level should never fall below 4 mg/L, otherwise the fish may be in serious danger.
How can you tell if your aquarium lacks oxygen?
It is impossible to tell with the naked eye whether water is low or rich in oxygen. So how can you tell if there is a problem?
Observe your fish. Their behaviour will tell you whether you need to test your water (we'll come back to this later). If oxygen levels are insufficient, fish slow down to conserve oxygen. Fish struggle to breathe: their gills move much faster than usual. Fish stay close to the surface because this is the area richest in oxygen, and they may even appear to be trying to breathe by opening their mouths at the surface. All of these behaviours indicate that your fish need more oxygen.
If you have different species, not all of them may display this behaviour. This does not mean that the problem isn't present; it simply means that some fish are more easily affected by low oxygen levels than others. It is still necessary to act to prevent the problem from becoming worse and the most vulnerable fish from dying.
Why can an aquarium become low in oxygen?
Overstocking!
By far the most common cause of low oxygen levels in aquariums is overstocking. An aquarium does not have unlimited capacity, and it is common to underestimate the needs of fish by keeping too many of them. Oxygen consumption then becomes disproportionate compared with the maximum amount the water can hold, meaning oxygen is consumed faster than it can be replenished. This problem is very common and worsens the other risk factors detailed below.
Plants!
As mentioned earlier, plants consume carbon dioxide and produce oxygen when exposed to light. However, this process changes when they no longer receive enough light, at which point they begin consuming oxygen. If you notice that your fish behave as though they are lacking oxygen when you turn on the lights, but this behaviour stops during the day, the issue may be partly related to the plants. Only partly, however, because plants themselves are beneficial to aquariums and cannot drastically reduce oxygen levels on their own. The underlying problem is therefore likely elsewhere, such as overstocking. The presence of plants can help reveal the underlying issue, so don't remove your aquarium plants!
High temperature!
The amount of oxygen in aquarium water depends, among other things, on temperature. The higher the temperature, the less oxygen the water can hold. During summer, aquarium temperatures can rise rapidly, especially if the tank is exposed to sunlight. As the temperature rises, which is already uncomfortable for the fish, oxygen becomes less available. If you notice that the temperature is too high, use the following methods to try to cool the aquarium:
Protect the aquarium from sunlight. If your aquarium has a hood, open it or raise it slightly. Turn off the heater if it is not equipped with a thermostat. In our guide on aquarium heating, we always recommend choosing a heater with a thermostat. This way, you won't need to turn it off, and it will automatically turn back on when the ambient temperature drops. Use a clip-on aquarium fan or, at minimum, direct a regular fan toward the surface of the aquarium. This is one of the most effective ways to lower the temperature, or use a fan specifically designed for aquariums.
Do not perform a water change using cold water. This would only expose your fish to thermal shock. If you have an external filter, you can place it in a container filled with very cold water, but replace the water regularly to maintain the temperature and avoid major fluctuations.
Use of chemicals!
Some medications can reduce the amount of oxygen available in an aquarium. If you need to administer medication to a fish, it is essential to place it in a hospital tank for treatment. Using medication in the main aquarium would affect all fish, including those that are not sick, and could completely destroy the bacterial population. The consequences can be very serious because the nitrogen cycle would have to restart. If oxygen levels are also low, this can become a recipe for disaster for those who are not prepared.
How to measure oxygen levels in an aquarium: the JBL O2 drop test.
It is essential to be able to test the oxygen level in your aquarium if you have any doubts. We have seen that the symptoms are generally clear enough to alert you to a problem, but testing the water remains necessary because these symptoms may indicate another issue. Fish can display the same behaviour (slow movements, rapid breathing, fish opening their mouths at the surface) for a reason other than oxygen deficiency: a nitrite spike.
In the case of a nitrite spike, the oxygen level in the water does not change. However, the fish's ability to absorb oxygen decreases drastically. The fish can therefore suffer from oxygen deprivation even when there is more than enough oxygen in the water!
So, low oxygen or a nitrite spike — are you unsure?
Run a test!