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. 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.

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. 

Causes of low oxygen include:
  • Overcrowding
  • Elevated water temperature
  • Lack of water movement
  • Excess waste
  • Low lighting with live plants
  • Use of certain chemicals
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.

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.

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.

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.

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. Obviously, fish need oxygen to breathe, plants need oxygen at night and the beneficial bacteria need oxygen to break down (oxygenate) waste. Basically everything that dies off or decays in the aquarium requires and therefore depletes oxygen. Unhealthy or dead plants, decaying live rock and live sand, and uneaten food just to name a few.

Organic acids, proteins, and carbohydrates can also reduce the oxygen level in the aquarium. Oxygen enters the water through gas exchange in two ways: Surface agitation Plant photosynthesis The amount of oxygen that can be dissolved (saturated) in the water is dependent on the water temperature and salinity levels. Increasing temperatures and salinity will allow less oxygen to be saturated in the water. For example, a freshwater aquarium with a temperature of 75 F can dissolve 8.4 ppm of oxygen. A saltwater tank with a temperature of 82.4 F and a salinity of 15 ppt can dissolve 7.18 of oxygen. The above levels show 100% saturation, the maximum possible. The average saturation in an aquarium is about 70%. The level of dissolved oxygen varies throughout the day, it is higher during the lighting period and has the lowest concentration in the early morning hours. Some reasons for low dissolved oxygen levels are: a short lighting period in planted tanks, plants produce oxygen during the lighting period and use up oxygen at night an overstocked tank, which translates to larger amounts of waste created that in return requires more bacteria to oxygenate the waste no or too little water agitation waste rotting in the filter or in the gravel Another term in connection with dissolved oxygen is the Redox (reduction-oxidation) potential. In simple words Redox refers to the relation of oxygen and waste particles. The more waste, the less Redox potential due to lesser oxygen. Redox can be measured in mV and the range should be between 150 to 250 mV. Redox is also related to the biological oxygen demand (BOD). BOD is the measurement of how much oxygen is needed to break down the waste created. The higher the BOD value, the worse the quality of the water. An acceptable BOD value is at about 1-2 mg/l (ppm). Nitrates contribute to high BOD levels, since this generally indicates a high break down rate of waste.