Insufficient Oxygen in Aquarium Water

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

Symptoms of Low Oxygen

Unfortunately, there is no flashing light or blaring alarm that goes off when dissolved 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 vigorously and even eat less often. As oxygen levels drop further, the fish will begin to show labored breathing and more rapid gill movements as they desperately attempt to get enough oxygen from the water by passing more water over their gills.

Eventually, 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 in some air at the surface, such as labyrinth fish. Certain species of fish, such as bettas, gouramis, and Corydoras catfish will periodically take a leisurely gulp of air from the surface. This is perfectly normal behavior for them, and these fish will not remain on the surface taking breath after breath. When any fish do go to the surface of the water for oxygen, they will gasp repeatedly, often with a wide-open mouth, sucking in the surface layer of water, which holds more oxygen.

Emergency Steps

If all of the fish are gasping at the top, the problem is critical and swift action should be taken. But action should be taken even in cases where only one single fish is 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 severely weakened by low oxygen levels.

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 temporarily adding a powerhead, airstones, or even an additional filter. The newly agitated water will introduce more oxygen to the aquarium, while the increased water movement will improve the oxygen exchange, buying some time to address the underlying cause. Next, the additional corrective steps will depend on the root cause of the low oxygen level, which should be determined promptly to assure the problem is permanently corrected.

Causes of Low Oxygen

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

High Water Temperature

Higher temperature water cannot hold as much oxygen as can water at colder temperatures. Performing a water change with lower temperature water will help to introduce fresh oxygen. A partial water change with dechlorinated water in the 65-70 degree F range will be safe for most tropical fish and help lower the aquarium water temperature. If the aquarium water becomes overheated, the heaters should be turned off, as well as lights. Remove the aquarium cover. Blowing air across the surface from a fan will also help to cool the water; it is wise to place a piece of screen over the top to keep fish from jumping out. Additionally, a few ice cubes placed in a zip-close bag can be placed in the tank to help drop the water temperature. Be sure not to lower the temperature below the fishes' normal temperature range.

Water Movement

Stagnant water 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 circulating sufficiently, that oxygen doesn’t reach the lower portion of the tank. It is important with water circulation to move the water at the bottom of the aquarium to the top, which displaces the aerated surface water down into the tank.

Filters go a long way toward increasing oxygen in the water, as they cause water movement at the surface where oxygen exchange occurs. Filters should take in water at the bottom of the tank, and release it back into the aquarium at the surface, thus distributing oxygenated water throughout. 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 filter cleaning.

More water movement always increases oxygenation. Add an additional filter or replace the existing filter with a higher capacity unit, if necessary. Other options are using a powerhead, putting a spray bar on the outlet of the filter, or using airstones. In a pond, the addition of a fountain will do wonders for aerating the water. Anything that moves the water at the surface, or splashes it through the air will increase oxygenation as well. In a pond, the water circulation needs to move all the water throughout the pond from bottom to top, to avoid low oxygen areas of stagnant water.

Excess Waste

Another common cause of low oxygen is often found in conjunction with overstocking. The more fish you have, the more waste they produce! Excess waste, clogged filters, and algae overgrowth all can cause decreased dissolved oxygen as well as a lowered oxygen-carrying capacity in the aquarium. Breakdown of detritus and waste by the bacteria in the aquarium gravel uses a large amount of dissolved oxygen. A thorough tank cleaning that removes debris from the gravel 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 cause of low oxygen in an aquarium. When they are exposed to light, all plants use carbon dioxide (CO2) and give off oxygen (O2). But when the tank or pond is dark the process reverses, and all plants including all algae will consume oxygen. If the aquarium has reduced or no light for a lengthy period, the plants or algae in the system could deplete enough of the oxygen to cause the fish to be affected. The obvious solution is to increase the lighting.

Chemicals

Some chemicals used to treat disease or modify water parameters can also impact the oxygen-carrying capacity of the water. Whenever using a chemical additive, always read product literature to ensure it doesn’t lower oxygen-carrying capacity. Always increase water circulation when medicating the fish. When troubleshooting an oxygen problem, discontinue the use of any chemicals that are not absolutely needed.​