Low Oxygen in Aquarium Water

Goldfish swimming in tank
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Low oxygen levels are rarely a problem if an aquarium is well maintained and not overstocked. However, if your fish are gasping at the surface of the water, it should set off a red flag prompting further investigation. Here is what to look for, and how to fix it.

Symptoms of Low Oxygen

There is no flashing light or blaring alarm that goes off when oxygen levels drop too low in an aquarium. Aside from actually testing the water for dissolved oxygen, the only indication of trouble will be the behavior of your fish.

GIF of the symptoms fish show in a tank with too low of oxygen
 Illustration: Alison Czinkota. © The Spruce, 2019

Fish will initially react to lower oxygen levels by simply moving around less. They will swim less and even eat less. As oxygen levels drop further, the fish begin to show labored breathing, and more rapid gill movement as they desperately attempt to get enough oxygen from the water by passing more water through their gills.

Eventually, the fish will begin gasping at the surface of the water. This surface breathing should not be confused with fish feeding at the surface or fish that can normally take some air at the surface, such as labyrinth fish. Certain species of fish, such as bettas and gouramis, will periodically take a leisurely gulp of air from the surface. That is perfectly normal behavior, and the fish will not remain on the surface taking breath after breath. When fish go to the surface of the water for oxygen, they will gasp repeatedly, often with a wide open mouth.

If all of the fish are gasping at the top, the problem is critical and swift action should be taken. Action should also be taken in cases where only some of the fish are gasping at the surface because eventually, the problem will get worse. Those that aren’t gasping for air are probably stronger fish or those that require less oxygen. If left unattended eventually they too will be affected by low oxygen levels.

Emergency Steps

The initial action to take is to perform a large water change of as much as 50 percent. At the same time, increase the water movement by adding a powerhead, airstones, or even an additional filter on a temporary basis. The newly added water will introduce more oxygen to the tank, while the increased water movement will improve the oxygen exchange, buying some time to address the underlying cause. After that, additional corrective steps will depend on the root cause, which should be determined to assure the problem is permanently corrected.

Root Cause of Low Oxygen

Overcrowding is the number one reason for low oxygen in an aquarium. In fact, other factors rarely cause fatalities by themselves if the aquarium is not also overstocked. That’s not to say the other factors should be ignored, but if the tank remains overstocked, correcting the other factors will not permanently resolve the issue. Causes of low oxygen include:

High Water Temperature

Warm water cannot hold as much oxygen as cooler water can. In warm times of the year, this can cause oxygen depletion in the water. Performing a water change with cooler water will help by lowering the temperature and introducing fresh oxygen. Heaters should be turned off, as well as lights. Remove the aquarium cover and blowing air across the surface from a fan will also help cool the water. It is wise to place a piece of screen over the top to keep fish from jumping out. A few ice cubes placed in a zip-close bag can be placed in the tank to help keep the water cooler.

Water Movement

Water that is stagnant will have lower oxygen levels. This is particularly true lower in the water column, where no oxygen exchange is occurring. Water at the surface will have more oxygen, but because it’s not moving, that oxygen doesn’t reach the lower portion of the tank. Filters go a long way towards increasing oxygen in the water, as they cause water movement at the surface where oxygen exchange occurs. Filters also move water from the top to the bottom of the tank, thus distributing oxygen throughout the aquarium.

Increasing water movement will increase oxygenation of the water. This can be done by adding an additional filter or replacing the existing filter with a higher capacity one. Before you do that, though, make sure your current filter is operating at full capacity. Often, the underlying problem is simply a badly clogged filter that is no longer moving much, if any, water through it. All that is needed in such cases is a good cleaning.

Adding a power head, putting a spray bar on the outlet of the filter, or using airstones will also help. In a pond, a fountain will do wonders for aerating the water. Anything that moves the water at the surface, or through the air, will increase oxygenation.

Excess Waste

This is another common root cause of low oxygen, often in conjunction with overstocking. Excess waste, clogged filters, and algae overgrowth all can cause decreased dissolved oxygen as well as lowered oxygen carrying capacity in the aquarium. A thorough tank cleaning will turn that around, and good ongoing maintenance will help prevent the problem from reoccurring.

Live Plants

Although it is not a common occurrence, live plants can be a root cause of low oxygen in an aquarium. Plants use CO2 and give off oxygen when exposed to light. When the tank is dark the process reverses and the plants use oxygen. If the aquarium has reduced no light for a lengthy period of time, the plants could deplete enough of the oxygen to cause the fish to be affected.

The obvious solution is to increase the lighting. Keep in mind that algae do the same thing as your live plants, so even if you don’t have live plants, this problem can occur in a tank that is heavily overgrown with algae.


Some chemicals used to treat disease or modify water parameters can also impact the oxygen-carrying capacity of the water. Whenever using chemical additives, always read product literature to ensure it doesn’t have a negative effect on oxygen capacity. When troubleshooting an oxygen problem, discontinue the use of any chemicals that are not absolutely needed.​