Lack of oxygen in the morning

The fish stay under the water surface in the morning, gasping for air. A little while after the light is switched on, the fish return to their normal swimming behaviour. We can thus eliminate some common causes, such as nitrite. If you notice this effect in the mornings or at night during the dark phase, this is a clear indicator of a lack of oxygen.

The reasons

But where does the lack of oxygen come from? First you need to ask yourself if any changes have been made to the aquarium or the stock over the last seven days, or if any new products have been used. The cause can often be found here. Extended cleaning measures, new fish stock or the use of a new care product are factors which already may lead to an oxygen deficiency. The strong consumption of oxygen during the day can still remain within reasonable limits and not be noticed, since the aquatic plants produce additional oxygen and counteract the effect. At night they come as consumers and so the content drops well below 4 mg/l.

In the following we list and discuss the different influencing factors which can lead to a lack of oxygen and which can help you find the reason in your aquarium:

Plants: One common cause, which is often underestimated, is having a high plant stock in the aquarium. During the day they produce large amounts of oxygen, but at night they "breathe" oxygen like fish and emit CO2. It is not unusual for Dutch planted aquariums and aquascapes to suffer from oxygen deficiency during the night. Aquariums in which more than 70% of the base area is planted especially tend towards this. This, however, should not discourage you from having lush plants in your aquarium, as they bring many benefits (see: Benefits of aquatic plants for the biosystem ).

Decoration: Using natural decorative elements, such as roots or leaves, can also result in heavy oxygen consumption. The fresher the organic material introduced, the stronger the effect. Bacteria pounce on the material and try to graze on the surface. The activity of the bacteria and the breakdown of the the nitrogen accumulated creates an increase in the oxygen consumption.

Algae: The strong growth or sudden appearance of algae in the aquarium ensures that nocturnal oxygen is consumed, as with plants. If the algae die off, for example by altering the water parameters (starvation) or by using an anti-algae agent, all of a sudden a large biological mass arises which has to be broken down by the bacteria. This process devours large quantities of oxygen.

High temperatures: In summer temperatures rise in the aquarium too. This has the effect that the oxygen absorption capacity of the water slowly decreases. With simultaneous high consumption, this change can already lead to the problem and needs to be remedied by lowering the temperature (more information can be found here: Caution, Summer Heat! A Reason for Scaremongering? ).

Bacteria: During the degradation process from NH4 to NO2 to NO3, the bacteria are working at full speed. The chemical processes here result in oxygen consumption. It is, therefore, not surprising that extensive cleaning measures, the disturbance of the bacterial culture and the running-in process of an aquarium, lead to an increased oxygen consumption. Any additional strain caused by new decorative elements, dying algae and dying aquatic plants (as mentioned above) therefore lead to an increased performance of the bacterial degradation processes and an enormous reproduction of the bacteria - this consumes oxygen.

Care Products: Before using a care product in the aquarium, including medications and anti-algae agents, always read the instructions carefully. These may contain important instructions for use in the morning or for additional ventilation. Often processes are triggered by ingredients which also affect the level of oxygen, for example, when the algae die off or by damaging the bacterial culture. This includes also a few filter media. That’s why attention should always be paid during this period to the morning dosage and a regular check needs to be scheduled for the next few days.

Fish: Fish breathe oxygen. A lot of fish breathe a lot more oxygen. In the many mails we receive, we see that very high overstocks are the most common cause of oxygen deficiency, along with low planting. Without additional measures (see the next section for more information) or without reducing the stocking density, the problem cannot be solved.

Feeding: When feeding, the fish snatch for food. But also for some time after that, increased gill activity due to heavy breathing is noticeable. Some fish repeatedly swim to the water surface and "gasp". However, this happens as part of the foraging and is not a sign of deficiency. Check the fish’s behaviour one hour later and if it is related to eating it will be gone.

How do you check the oxygen content?

With the JBL O2 kyslíkový test New Formula you can check the oxygen content of your aquarium water. In just 30 seconds and with 2 x 6 drops of reagent, after comparing the colour on a colour chart, you’ll have the result. The oxygen content can be determined in a weekly check before a water change or in case of acute problems at the times described below. Write down the results and compare the values at the end:

  • In the morning, half an hour before turning on the light
  • During the day, when the lighting has been on for at least 3 hours
  • In the evening, half an hour before the light is turned off
  • At night, when the light is off for at least 3 hours

This way you can quickly see if a nocturnal oxygen deficiency explains the gasping for air in the morning.

The solution

In the previous sections, you learned how to detect an oxygen deficiency, what causes it, and what steps are needed to analyse it. If there is an acute need for action, the following measures can be taken to remedy the situation:

Modifying the filter outlet: Definitely the easiest option is to position the filter outlet of your internal or external filter in such a way that the current moves the water surface strongly. Position the outlet below the water surface and direct it diagonally upwards.

Installing a diaphragm pump: The active aeration of the aquarium with a diaphragm pump (e.g. JBL ProSilent a200 ) not only ensures an additional circulation of the water but also brings fresh oxygen from the air into the water, which enables an improvement within a few minutes. For nocturnal problems, it is advisable to run the diaphragm pump during the dark phase. Some filters offer the possibility of installing diffusers ( JBL VenturiSet 12/16 ) at the outlet, which also introduces air into the water.

Oxygen tablets: Oxygen tablets, such as JBL OxyTabs , are not designed to solve oxygen problems in aquariums. They provide oxygen enrichment for a short time only. They are used when no technical items or electricity are at hand, for example when transporting fish or when putting them into a bucket for a short time.

Increasing the plant stock: As a long-term solution, it can also help to insert additional aquatic plants into the aquarium and thus enter oxygen into the water in a natural way, namely through photosynthesis.

Oxydators: Without further technology but with the use of chemical products, you can continuously increase the oxygen content in the aquarium using so-called oxydators.

Remedy the causes: In addition to all the countermeasures mentioned above, do not neglect to rectify the root cause and thus prevent a further occurrence of the problem. In addition to newly introduced decorative elements, products in use and active treatments, you’ll need to check the aquarium for "rotting areas" and to remove dead plant parts, algae and sludge. This reduces the workload of the bacteria and thus has a lasting effect on reducing the problem.


In many requests for help hobbyists reported a suspected lack of oxygen. After a short analysis, it usually turned out that it was typical nitrite problems that occurred due to an overstocking or a damaging of the bacterial culture. For this reason it’s best to go through these points step by step until you have learnt to differentiate between the causes and can determine the one in this case.

Use the comment function if you can think of other aspects or if you would like us to go deeper into a subject area for you.

© 06.09.2018
Matthias Wiesensee
Matthias Wiesensee
M.Sc. Wirtschaftsinformatik

Social Media, Online Marketing, Homepage, Kundenservice, Problemlöser, Fotografie, Blogger, Tauchen, Inlineskating, Aquaristik, Gartenteich, Reisen, Technik, Elektronische Musik

O mně: Seit Teenagerzeiten mit Aquarien in Kontakt. Klassische Fischaquarien, reine Pflanzenaquarien bis hin zum Aquascape. Aber auch ein Gartenteich und Riffaquarien begleiten mich privat im Hobby. Als Wirtschaftsinformatiker, M.Sc. bin ich als Online Marketing Manager bei JBL für die Bereiche Social Media, Webentwicklung und der Kommunikation mit dem Anwender der JBL Produkte zuständig und kenne die JBL Produkte im Detail.


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