Low Oxygen in Aquarium Water

Oxygen is essential in the life of aquatic animals. Fish, shrimp, freshwater snails, crabs, etc. need oxygen for their metabolic processes. It also keeps them in optimal condition, and if they are deprived of oxygen for long— they will die.

Just like other living animals, they breathe in oxygen and releases carbon dioxide. The distinction here is that oxygen is less available to aquatic organisms e.g. fish than it is to land animals. Also, when for example, fish breathe dissolved oxygen i.e. oxygen that is dissolved in water, and they can only absorb this dissolved oxygen in the water using their respiratory organ— gills, whereas land animals that are capable of absorbing oxygen from the atmosphere using their lungs.

Unfortunately, the low oxygen level is detrimental to the health of any aquatic pet. It will limit their activities and prompt them to swim or crawl towards the water surface to gasp for air to stay alive.

Keep reading for in-depth information on effective ways of detecting low oxygen levels in fish and shrimp tanks, causes, and how to remedy the problem.

Symptoms of Low Oxygen in Aquarium Water

While it may be easier for one to test the aquarium water for dissolved oxygen, there are some signs that hint at low oxygen levels in the tank and they mostly center on the behavior of your aquarium fish and shrimp.

When the dissolved oxygen level is lower than normal, the fish may respond by exhibiting slow movements and swimming less, while some may suddenly stop feeding or will eat less often.

As with dwarf shrimp, they are also pretty sensitive to low oxygen levels. In most cases, it is possible to notice:

1. Staggering swimming behavior. They may swimming fast and then got paralyzed mid-air and going down and swimming again.
2. Complete inactivity, they stop moving.
3. shrimp lay on their side.

As the oxygen level continues to drop even further, your fish will have trouble breathing air within the tank, hence they may swim towards the surface to take gulps of air.

Keep in mind that some species of fish like Bettas and Gouramis are already used to breathing air from the surface. It’s a normal activity for them. So their occasional trips to the water surface to breathe should not be mistaken as an indication of low oxygen levels.

However, this is not the case with shrimp. Unlike fish, shrimp usually do not try to reach the surface making it hard to figure out the problem fast enough.

In a situation of low oxygen, the fish will swim to or near the surface to take several gulps of air with their mouths open. It is important to mention that sensitivity to low levels of dissolved oxygen is species-specific, however, most species of aquarium fish would be distressed and lethargic when dissolved oxygen drops to 2-4mg/l (5mg/l and above is considered to be optimal).

Moreover, larger fish are affected by low oxygen levels before smaller fish. So if you notice larger fish at the surface more often while the smaller fish are busy swimming, carry out an oxygen test to ascertain if dissolved oxygen is at the right level.

Besides, a closer look at the gills of your gills will reveal their desperate attempts to absorb enough oxygen from the aquarium water by pushing water through their gills at a faster rate.

Causes of Low Oxygen in the Tank

Here is the list of the main causes of low oxygen in the tanks:

1. Overstocking
2. High Water Temperature
3. Water Movement
4. Excess Waste
5. Chemicals and Medications
6. Aquatic Plants
7. Driftwood and Biofilm

1.     Overstocking:

This is the most common cause of oxygen depletion in community tanks.

Keep in mind that is difficult to maintain good water quality in an overstocked/overcrowded aquarium. If an aquarium is overstocked, the oxygen in the water would be consumed faster than it can be replenished.

Having too many fish in the same tank can be quite unpleasant— with numerous fish depositing waste into the aquarium and competing for the same oxygen supply.

That is why hobbyists should adhere to stocking limits, perhaps following the rule of one gallon per one inch of adult fish. Make sure not to keep more fish than your tank can contain, and always consider the volume and surface area of your tank while stocking.

Note: Shrimp only tank usually do not have this problem because dwarf shrimp do not require a lot of oxygen to breathe.

2.     High Water Temperature:

Warm water is less capable of holding as much oxygen as cool water. For instance, aquarium water that has a temperature of 30 °C (86°F) can only hold 7.6mg/l of dissolved oxygen whereas the water of 15 °C (59 °F) can hold 10.1mg/l of dissolved oxygen.

With the cited example, you can see that water will hold less oxygen as the temperature increases and vice versa.

Therefore, the water temperature should not be allowed to rise too high to prevent the level of dissolved oxygen from falling, and you should be aware that high water temperature will trigger a rise in the metabolism of fish and the demand for more oxygen.

When faced with a low oxygen situation, you should perform a water change with water of lower temperatures to instantly introduce fresh dissolved oxygen in the aquarium.

Alternatively, turn off the heater and install an aquarium chiller or you can blow air across the water surface using a fan to cool the water temperature and promote oxygen concentration.

Another effective method is to place a zip-close bag with ice cubes in the tank to cause a decline in the temperature. Likewise, shade the aquarium from sunlight to prevent the temperature from rising steadily.

3.     Water Movement:

This also has an impact on the oxygen concentration of aquarium water. It is essential to constantly agitate the water surface as it increases the gaseous exchange at a sufficient level and encourages the healthy living of aquatic creatures.

The oxygen present in the tank water needs to be circulated from the surface to lower regions and dead spots for it to be distributed throughout the tank. For this purpose, an aquarium filter and other bubble-producing decors should be placed in the aquarium to aerate the water, thus keeping it properly oxygenated at all times.

Absolutely still water is not ideal for most tank setups. So make sure to install a good filter to increase dissolved oxygen. Ensure that the aquarium filter is operating at full capacity and always carry out regular maintenance to keep the filter clean as it may get clogged with dirt from time to time.

Apart from an aquarium filter, other powerful aerators like powerhead and wavemaker may be needed in certain situations to create even more water movement.

Besides, spray bars, air stones, and bubble walls are also good at agitating the surface of your aquarium and increasing the dissolved oxygen level. On the other hand, one can use a fountain to increase the oxygen levels in ponds.

4.     Excess Waste:

It is worth noting that excess waste also contributes to oxygen depletion in aquarium water.

• Fish, crayfish, crabs, and snails produce a lot of bioloads (waste),
• Decaying plant matter,
• Dead or decomposing fish/inverts
• Uneaten fish food

are all capable of polluting the tank water and altering the water quality thereby leading to algae and bacterial blooms which can also deplete the water of all its dissolved oxygen.

A properly maintained aquarium filter in addition to regular cleanings will help to rid the aquarium of excess waste and nitrates, hence keeping it clean.

5.     Chemicals and Medications:

This is another factor that affects the oxygen level of water in an aquarium. Certain medications used for the treatment of fish and shrimp diseases, and chemicals capable of modifying water parameters can cause oxygen levels to drop significantly by affecting the ability of aquarium water to hold oxygen.

Therefore, it is important to always confirm whether the chemical additives you use for your tank contain properties that can lower the oxygen level.

Moreover, it is advisable to discontinue the use of chemicals when dealing with low oxygen issues in an aquarium.

6.     Aquatic Plants:  

Aquatic plants greatly benefit our tanks by promoting oxygenation through photosynthesis. However, in some situations, they can also become the cause of the problem.

Regardless of their role in oxygenation, live plants may also contribute to low oxygen levels in your tank, and here’s how:

Live plants use up carbon dioxide in the water and liberate oxygen as a by-product of photosynthesis. However, if there is no sufficient lighting for them to carry out photosynthesis— the plants together with fish and other organisms present in the tank will consume the available oxygen, and if this goes on for long, they could deplete enough oxygen to affect the life of your fish.

Hence, it is essential to provide ample lighting in the aquarium to enable the plants to produce oxygen, thereby boosting the dissolved oxygen level of the tank water.

Another problem is that if you do not have any aeration in the tank or there is no water flow that can disturb the surface, some floating plants can create a thick layer on the surface for a long time. It can result in oxygen depletion in the water column by reducing the gas exchange. It may potentially lead to the suffocation of your fish or shrimp.

You can also read my article “Aquarium Floating Plants. Pros and Cons”.

7.     Driftwood and Biofilm

Even though biofilm is one of the favorite foods for shrimp and it plays an important part in their menu, it can also play a big role in depleting oxygen in the tank. It can be especially important for surface biofilm.

Basically, it may cause a rapid increase of CO2, which will lead to an asphyxiation of the fish, shrimp, etc. In the worst-case scenario, excess of biofilm can also suffocate the nitrifying bacteria and completely crash the nitrogen cycle.

Driftwood is one of the best surface areas for growing biofilm. So, be careful with that.

Read my articles:

Biofilm In Our Aquariums. How to Remove
Driftwood in Shrimp Tank

Emergency – Low Oxygen in the Tank. What to Do?

In a fatal situation where you notice your fish steadily gasping for air (suffocating) or constantly making trips to the surface to breathe due to low dissolved oxygen in the tank, act immediately!

Trying to ascertain the cause of the problem at this point may be telling and time-consuming, so you should focus on saving the life of your dear pets first and troubleshooting the problem later on.

1. Start by changing a large volume of the aquarium water— replace about 50%, the oxygen level would be given a boost instantly.
2. Afterward, increase the water movement by adding a powerhead, spray bar, or air stones, this will break surface tension and promote gaseous exchange in the aquarium.
3. Another good idea is to replace the current filter with a bigger model or installing an additional filter for more aeration. At this juncture, you have succeeded in oxygenating the tank and saving the life of your fish, you can now tackle the main cause of the problem permanently to forestall future occurrence.

Too Much Oxygen in an Aquarium

Be wary of a very high oxygen level in the aquarium as it is also detrimental to the health of your aquatic friends.

High oxygen level leads to a lethal gas bubble disease, whereby bubbles begin to form on the fish’s skin, eyes, and fins, and this condition may worsen further resulting in its demise.

Testing the Oxygen Levels

The dissolved oxygen content of aquarium water can be measured using an oxygen test kit (check the price on Amazon) or an oxygen meter (way too expensive!).

Make sure to monitor the oxygen levels regularly, keeping in mind that a freshwater aquarium at 77 °F should hold about 8.3mg/l of dissolved oxygen while that of a saltwater aquarium of the same temperature should be about 6.6mg/l.

An oxygen meter delivers a more accurate measurement than a test kit, although you are required to swap its batteries periodically and to calibrate the meter at intervals for optimum performance.

In Conclusion

Hobbyists should pay serious attention to the dissolved oxygen content of water in their tanks to encourage good health and survival of aquatic species.

Variables such as elevated temperature, excess waste, and addition of substances are capable of affecting the oxygen-carrying ability of tank water, thus resulting in oxygen depletion which can cause distress, labored breathing, and mortality of your pets.

One should strive to keep the aquarium water clean by performing regular water changes and engaging in routine tank cleaning and maintenance.

Furthermore, ensure you provide ample lighting, invest in a quality filter and bubble-producing decors, and also monitor the dissolved oxygen levels regularly to prevent it from falling to critical values.