6 Causes of Low Oxygen and Ways to Increase Oxygen in a Fish Tank

If you notice low oxygen in your fish tank, there are some easy solutions.

Reviewed by Nick Saint-Erne

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.

The average aquarium fish will not survive more than a day in an oxygen depleted tank. Here is what to look for and how to remedy the problem of low oxygen.

How Much Oxygen Do Fish Need?

The optimal amount of oxygen in a fish tank varies according to the species of fish you have and the atmospheric pressure of your home's location (the higher the altitude, the less oxygen water can hold).

Fortunately, you don't need to calculate an exact amount based on these parameters. If your fish appear active and happy, and none are gulping air from the surface, your tank's oxygen level is sufficient

If you would like to measure the exact parts-per-million (ppm) of dissolved oxygen in your aquarium, purchase a portable dissolved oxygen meter. A generally safe oxygen level for all freshwater fish is 8.3 ppm.

Signs 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 moving around less. They will swim less vigorously and even eat less often. As oxygen levels drop further, the fish may 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 species that normally "breathe" at the surface, such as labyrinth fish.

6 Causes of Low Oxygen and How to Fix It

If your fish are gasping at the surface of the water, take emergency action to immediately increase the tank's oxygen:

Perform a large water change (as much as 50 percent of the tank's volume). Then increase the water movement by adding a powerhead, airstones, or even an additional filter. These measures will introduce more oxygen to the aquarium, buying some time to address the underlying cause.

Overcrowding

Overcrowding is the number one reason for low oxygen in an aquarium. The rule of thumb for stocking a tank is one inch of fish per one or two gallons of water.

High Water Temperature

Higher temperature water cannot hold as much oxygen as cooler water. If the temperature in your tank is too high, turn your heater down (or off, temporarily). Performing a water change with water in the 65 to 70 degree Fahrenheit range will introduce fresh oxygen and safely lower the water temperature. 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 ideal temperature range for your fish.

Water Movement

Stagnant water has low oxygen, so aquarium water must be agitated or moved regularly to maintain adequate oxygen. Filters help increase oxygen because they move water, so make sure your current filter is operating at full capacity. A clogged filter must be cleaned or replaced. In addition, you can install a spray bar on the outlet of the filter or add air stones to the tank.

Dirty Tank

Accumulated fish waste and algae overgrowth can sap oxygen from an aquarium as bacteria work to break down the excess organic matter. A thorough tank cleaning to remove debris from the gravel and tank walls will boost oxygen.

Live Plants

Occasionally, live plants can deplete oxygen in an aquarium. When exposed to light, plants absorb carbon dioxide (CO2) and release oxygen (O2). When the tank is dark, the process reverses, and your plants—including algae—will consume oxygen. Adding a light or increasing the lighting duration during the day can help raise oxygen levels.

Chemicals

Some chemicals used to treat fish diseases or modify water pH can also impact the oxygen level. If you need to use a chemical additive, err on the side of safety by increasing the tank's water circulation for the duration of treatment so that it maintains sufficient oxygen.