How To Increase Oxygen In A Fish Tank


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Increasing water movement is the quickest way to increase oxygen (O2) levels in a fish tank, as it allows more O2 to dissolve and carbon dioxide (CO2) to be released. This can be easily done using an air pump, performing large water changes, manually stirring the water, or placing a fan near the aquarium. 

Oxygen, in particular, dissolved oxygen (DO), is one of the most important indicators of water quality, as it is essential for the survival of fish and other aquatic organisms. 

When DO levels drop too low, aquatic life cannot survive. Therefore, if your fish tank ever suffers from oxygen depletion, it is critical to quickly increase the oxygen levels. 

Where Do Fish Get Oxygen From In Fish Tanks?

Did you know that the oxygen your fish breathe in fish tanks, is not directly sourced from the water? 

The main supply of oxygen (O2) in fish tanks comes from the surrounding air. This is because oxygen naturally found in water is inseparable, therefore your fish cannot remove the oxygen atoms from the hydrogen to utilize it. 

Instead, your fish rely on gaseous exchange, which takes place at the water surface; carbon dioxide (CO2) from the aquarium water and O2 from the surrounding air is exchanged. 

Oxygen is also dissolved into aquarium water via increased aeration (water movement) and photosynthesis from live plants. 

How Much Oxygen Do Fish Need In Fish Tanks?

Oxygen requirements vary between fish species, however, it is recommended your aquarium water has an 80-110% oxygen saturation and DO level of 6-8 mg/L. 

Supersaturation (anything >115%) in aquariums should be avoided, as it is one of the causes of gas bubble disease in fish. 

What Causes Oxygen Deficiency In Reef Tanks?

  • Overstocking: Too many fish will result in a shortage of O2. 
  • Elevated water temperatures: Water with high temperatures, cannot hold as much oxygen as colder water. 
  • Low water movement: Sufficient water flow is required to help oxygenate/aerate fish tanks due to the absence of waves. 
  • Excess waste accumulation: Overstocking, overfeeding, and poor tank maintenance and cleaning can drive increased ammonia and phosphate levels, which lowers O2 levels. 
  • Low lighting environment: If your reef tank has poor lighting and live plants, your plants will start utilizing the O2 in the water instead of CO2, and therefore, the plants will release CO2 into the water instead of O2.  
  • Using certain chemicals and medicines: Some products can deplete oxygen levels. 

Symptoms Of Low Oxygen Levels In Fish Tanks

When O2 levels drop too low, no alarm bells will start ringing to notify you. Besides measuring O2 levels with an oxygen sensor, the only indication of low O2 levels in your fish tank is the behavior of your fish. 

So, what should you look out for in your fish?

  • Labored breathing 
  • Rapid gill movements
  • Gasping at the surface*

If you notice any of the above symptoms, it is important to measure the amount of O2 in your fish tank with a dissolved oxygen probe/sensor immediately. Instructions on how to use a DO probe can be found here

*Gasping at the surface must not be confused with the natural behavior of labyrinth fish. Fish such as Bettas, Gouramis, and bottom-feeder Catfish will regularly swim to the surface and gulp air using their labyrinth organ. This doesn’t take too long, so if they are spending most of their time at the surface, it is likely your fish tank has low O2 levels.

Ways To Increase Oxygen In A Fish Tank 

1. Pouring Water From A Height

The most simple way to increase O2 and help aerate water in a fish tank is to slowly pour water from a height. 

Take a small container and fill it with some aquarium water. Gently pour the water out into the fish tank from a height to create movement. 

2. Large Water Changes

A water change up to 50% will help pass large amounts of oxygenated water into your fish tank. 

3. Stirring The Water Manually

Another excellent way to increase O2 levels is to manually stir the water. This forces movement inside the fish tank, increasing aeration. 

4. Adding Ice Cubes & Using A Fan

If your fish tank is suffering from low O2 levels due to rising temperatures, adding ice cubes will help cool the water down. When adding ice cubes to your aquarium, always place them in an airtight bag, so they do not melt into the aquarium water, which could cause fluctuations in your water parameters.  

Placing a fan near the top of the fish tank can also help cool the water, plus it creates ripples on the surface, increasing the water movement. The water movement will help oxygenate the surface water.

Remember to monitor temperature levels with a temperature sensor if you are performing water changes or blowing a fan to increase O2 levels. 

5. Using A Battery-powered Air Pump

Often, power cuts happen, especially if you live in a storm-prone area. Unless you have invested in a home generator, any electricity connected to your fish tank will also go out if the power is cut. A battery-powered air pump is a great way to keep water moving inside your fish tank, should a power outage occur. 

Long-term Ways To Increase Oxygen In A Fish Tank 

The above ways to increase O2 in your fish tank are great in emergencies, however, to prevent low O2 levels from happening again, we have some more permanent solutions to coping with O2 depletion. 

1. A Hang-on-back (HOB) Filter

A hang-on-back, or HOB filter, is the most common and widely-used aquarium filter. Not only are they a great way to increase O2 levels inside fish tanks, but they are also very small and easy to install. 

Filtered water is released from the HOB filter, which aerates the water and evenly distributes it throughout the fish tank. 

2. Spray Bars

If you have a canister filter, you probably already have your hands on a spray bar, as almost all canister filters have them. So, if your O2 levels have suddenly dropped, and you didn’t know you had one, now is a good time to use it. If your canister filter did not come with one, do not worry, as you can easily buy them online. 

Spray bars work by distributing filtered water around the entire fish tank, therefore, increasing the O2 content of the water, preventing drops in O2.  

3. Air Pumps 

As the name suggests, air pumps are designed to pump your fish tank with air (O2). Air pumps utilize air stones to collect and release O2 inside the aquarium water, making them a straightforward way to increase oxygen in your fish tank. 


Pouring water from a height, manually stirring the water, or performing large water changes, are some ways to quickly increase oxygen in fish tanks. These methods allow more oxygen to dissolve in the water, and release carbon dioxide. 

More permanent methods such as using spray bars, HOB filters, and air pumps should prevent oxygen levels from dropping again.

If you have any questions regarding oxygen/dissolved oxygen, or what O2/DO sensor would best suit your testing needs, please do not hesitate to contact the world-class team at Atlas Scientific, we are always happy to help! 

Dissolved Oxygen Probes & Sensors

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