An overview of how to identify signs of low oxygen in your aquarium water, potential causes and how to fix it.
Common Signs of Low Oxygen in Fish Tanks
Fish, plants, and beneficial bacteria all rely on dissolved oxygen in the water to survive. Proper oxygen levels not only support respiration but help to maintain important biological processes like the nitrogen cycle in your tank.
Oxygen levels in your fish tank should typically sit somewhere around 8 ppm, for a freshwater community tank. Changes in fish behaviour can be an early warning sign of when these levels are beginning to drop. These signs will change with the decrease in the amount of available oxygen content of the water.
Initial signs of low oxygen are often less obvious. These inlclude reduced activity and lethargy of your fish. This is often displayed by slower, and less frequent movements about the tank, and they might also show less interest in eating their food.
As oxygen levels continue to decrease, you will likely witness rapid and exaggerated gill movement, as the fish start to breathe more frequently to try to increase their uptake of oxygen from their surrounding environment.
When that alone becomes insufficient, often, fish move higher in the water column where there is a greater gradient of oxygen transfer between the water and air, and thus more available oxygen. This tends to become more noticable, when bottom-dwelling fish start moving higher up in search of better oxygen levels, and, ultimately, if you see fish gasping at the water’s surface, appearing to gulp air. This behavior indicates they are struggling to extract enough oxygen from the water.
In more servere cases, prolonged oxygen deprivation will lead to fish death (<~2 ppm). This will typically happen in more sensitive species first, or those who are already weakened from age or illness. Fish can recover from oxygen deprivation if oxygen levels are resupplied quickly enough.
Causes of Low Oxygen Levels in Fish Tanks
Drops in oxygen levels in your aquarium water can occur both gradually and suddenly. Instances of a sudden drop in oxygen will typically require a faster correction by you, to keep your fish alive and healthy.
A common cause for this is a failed or failing water filter, causing poor water circulation and stagnant water. Lack of proper water movement prevents oxygen from being evenly distributed throughout the tank, and stagnant areas in the tank can quickly become oxygen-deprived. Check that your filter is running correctly, and that it is generating ripples/surface agitation near the outlet value.
Other situations include when oxygen is being depleted faster than it can be replensished. Causes of this can include, overcrowding in the tank and excess organic waste and its decomposition. Excess food, decaying plants, and fish waste all contribute to oxygen depletion as bacteria break them down, consuming oxygen in the process.
High water temperature can be another culprit. Warm water holds less dissolved oxygen than cooler water. A temperature increase of just a few degrees can significantly reduce oxygen availability. For example, an increase in temperature from 24 C (75.2 F) to 26 C (78.8 F) will decrease oxygen sauration in the water by ~4.7 %, or 0.4 ppm.
It is also important to keep in mind the daily fluctuations in oxygen levels that you tank might experience. Normally, these are not something to worry about, but if your fish are already struggling with low oxygen, then these further flucations could exacerbate the issue.
The first, in terms of environmental temperature on your tank – if it is unheated – or if there’s any times in the day when it is exposed to direct sunlight. The second,
The second will be experienced if you have a planted tank. Plants produce oxygen during the day through photosynthesis but consume it at night. Therefore, in heavily planted tanks, oxygen levels can drop significantly overnight if there’s not enough aeration to offset this nightly event.
How to Test for Low Oxygen in Fish Tank Water
Beyond observing the behavour of your fish, as discussed above, the best way to know how much oxygen is dissolved within the water of your aquarium is to directly test it using a Dissolved Oxygen Test Kit. These kits allow you to directly measure the concentration of dissolved oxygen. They’re a reliable way to determine if your tank’s oxygen levels are in the optimal range. As with other water test kits, these can come in strip, liquid and electronic meter forms.
How to Increase Oxygen in a Fish Tank
Oxygen enters the water primarily through surface agitation, where it gas exchanges with carbon dioxide occurs. If you suspect, or measure, low oxygen levels, the fastest way to add oxygen is to improve surface agitation. And, the larger the surface area of the tank, the faster that exchange can happen.
This can be done using filters or powerheads to create water movement at the surface, and also throughout the tank. If needed, raise the level of the outflow valve to further exaggerate this. You can even get it to break the surface, adding water bubbles to the tank as they flow re-enters. Alternatively, you can use an Air Pump or Air Stone. Air stones break the flow into lots of smaller bubbles, whican can further enhance surface agitation.
A rapid, but temporary, fix can be to do a partial water change in the tank. Thus, adding new oxygenated water in the aquarium for your fish. Note, this alone will not be a sufficient solution, but can buy you additional time to fix the problem if the above suggestions are not an option.
Longer term solutions to persistent, low oxygen levels can include reducing water temperature in the tank (consider the ranges that your fish can survive at for this and change the temperature gradually), removal of any decomposing organic waste from the tank, and adding more live plants, particulaly species that as fast growing, like hornwort or anacharis, to boost oxygen levels naturally.
