Signs of Low Oxygen in Fish Tanks (and How to Fix It)

Posted on July 03 2024, By: Aquarium Dimensions

Your fish is gasping for air near the surface of the aquarium. Is this a sign of low oxygen levels in the tank? Let's take a look.

How to Tell if Oxygen is Low in an Aquarium

Run a test for oxygen levels and take careful notice of your fish

The first step is to get a testing kit, such as API Master Test Kit, to measure the amount of dissolved oxygen in the fish tank water. This isn't a silver bullet, though. The results can show a low oxygen level, but it can't diagnose the underlying cause. You need to also observe the behavior of your fish for more clues in relation to the measured level. 

In the beginning of low oxygen levels, fish become sluggish. They will swim less and eat less. As the level of oxygen drops further, the fish start to show labored breathing and rapid gill movement, a sign of their desperate attempts to breathe more oxygen. In time, the fish will start gasping at the water surface.

This behavior should not be confused with feeding fish at the water's surface. Some species, such as labyrinth fish, normally take air at the water surface, too. Gouramis and bettas also periodically take a leisurely gulp of air at the surface of the water. This is a normal behavior, and it's only abnormal if the fish stays at the surface gasping for air. 

What starts as one fish gasping will turn to two or three, and eventually all the fish in the tank will be at the surface.

It's time to take emergency steps.

Emergency Intervention: Change 50% of the Water & Add Oxygen-Loading Accessories

The first step you should take is to do a large water change--as much as 50 percent. Simultaneously, increase the movement of water by adding a powerhead, airstones, or an additional filter. The new water will offer more oxygen to the fish tank, and the increased water movement will enhance the exchange of oxygen. This buys you time to find the underlying cause.

Now it's time to play detective. Why are oxygen levels low? 

Low Oxygen in Fish Tanks: Causes and Solutions


Overcrowding is the number one cause of low oxygen in a fish tank. A fish tank of a certain size is suitable enough for a certain number of fish. If you have too many fish in an aquarium, it becomes a fight for oxygen. 

The solution to this issue is to reduce the number of fish in the tank. 

Elevated Water Temperature

Warm water hold less oxygen than cold water. This becomes an issue during the warmer months of the year and tank temperature isn't as easily controlled.

The first step is a water change of 25 to 50 percent using cool water. This will lower the temperature and introduce fresh oxygen.

Additionally, heaters and lights should be turned off. Blow air from a fan across the water surface. A closed bag containing ice cubes can also be placed in the fish tank for lowering the water temperature.

Be sure your thermometer is in good condition, and closely monitor the temp. 

Excess Waste

Excess waste is another common cause of low oxygen levels, usually in conjunction with overcrowding. Excess waste, algae overgrowth, and clogged filters can all result in low oxygen.

The solution to this is a thorough cleaning of the tank, as well as ongoing maintenance to reduce the chances of this problem in the future. 

Stagnant Water

Stagnant water has a low level of oxygen. This is particularly the case near the bottom of the tank, as there is no natural exchange of oxygen independent of outside force.

Solve this issue starting at the filter. Service your existing one or install a new filter. Filters increase oxygen in the tank by moving water to the surface, where the exchange of oxygen takes place. The filter also moves water from top to bottom, distributing oxygen throughout the tank.

Live Plants/Too Little Light

Though it is not common, live plants can be a cause of low levels of oxygen in a fish tank. We all know that during the day, plants take in carbon dioxide and release oxygen. When the fish tank is in dark, though, the process is reversed; plants release carbon dioxide, which can make it harder for your fish to breathe.

The solution to this problem is to increase lighting. You can also remove some live plants from the tank.

Harmful Chemicals

Chemicals used to treat fish disease or water can also have an impact on the oxygen capacity of the tank.

The solution is to be careful when adding chemical. Follow the instructions provided with the chemical. Eyeballing a measurement can have unintended consequences.


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