Many marine aquarists only give the oxygen (O2) content in their aquarium water a passing thought, not fully understanding how the level of the Dissolved Oxygen (DO) in their aquariums can affect their entire tank population.

Everything Reacts With Oxygen

In one way or another, everything (fish, invertebrates, corals, seaweeds, bacteria) in the oceans and in marine aquariums react with oxygen. Most of the living beings of this planet require oxygen in order to metabolize nutrients, the principle byproduct of this reaction is carbon dioxide (CO2). Seaweeds (saltwater plants) absorb CO2 and expel O2 during the daylight hours, reversing the process at night, absorbing O2 and expelling CO2 in the dark. Fish, invertebrates and aerobic bacteria absorb O2, expelling CO2 day and night.

Organic Materials Use Oxygen, Too

Your critters are not the only things that use oxygen in your tank. Organic materials (Dissolved Organic Compounds), such as that stuff on a piece of uncured live rock or the uneaten food and fish detritus on the bottom of the tank, consume a lot of oxygen as they break down. There are some nitrifying bacteria additives that will multiply very rapidly to consumes wastes in the aquarium that require a large amount of oxygen in the water. When using these additives, make sure that extra aeration is added to the tank.

Saltwater and Oxygen

Saltwater has a lower capacity (saturation level) to hold O2 than freshwater does. The amount of O2 that saltwater can contain is dependent on the temperature and salinity level of the water. As you can see in the chart below, the lower the temperature and salinity level, the more oxygen the water can hold. Depending on species and stage of life, it is believed that a dissolved oxygen content of 5-6 parts per million (ppm = mg/L) is sufficient for most aquarium occupants. The first signs of stress will show if the content drops below 3-4 ppm and fatalities can be expected below 2 ppm. A Dissolved Oxygen Test Kit is inexpensive and easy to use. Testing your saltwater periodically is a good idea, particularly when the livestock load is increased, live rock is added or when the biological filter capacity has been increased or decreased.

Reasons for Low Dissolved Oxygen Levels

  • Overstocked aquarium (fish consume oxygen and produce waste, which also consumes oxygen as it is broken down by bacteria).Filters and substrate clogged with fish waste and uneaten food (results in decreased water flow and oxygen to the aerobic bacteria in the biofilter).Poor gas exchange at the water’s surface (that film of protein on the surface really slows gas exchange).

A vast majority of gas exchange (O2 in, CO2 out) in an aquarium takes place at the water’s surface. Vertical movement of water in the tank greatly increases gas exchange. This can be accomplished with powerheads that point to the water surface, aiming of the filter outlets up, or installing airstones (that bring water from the bottom to the top with the bubble movement). While airstones are a great way to move water vertically in an aquarium, their bubbles bursting at the water surface are also the major cause of salt creep, which greatly adds to maintenance woes. Protein skimmers are also an excellent method for increasing the oxygen levels in marine aquarium water.

Salinity is measured in Parts per thousand (ppt) or Specific Gravity (SP) and the dissolved oxygen content is measured in parts per million or milligrams per liter (mg/l = ppm).

  • Water Quality Test Kit Instruction Manual. Maine Department of Environmental Protection.

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