Oxygen – Quick Reference

Oxygen – Quick Reference

A readily available source of oxygen (O2) is as critically important to life underwater as it is for life on land. Like terrestrial animals, aquatic animals require oxygen for respiration, metabolism, fluid transport and energy.

Oxygen is present in the air at about 20% and is dissolved in water even at the greatest ocean depths. The amount of oxygen dissolved in water is measured in parts per million, ppm. Water absorbs oxygen from the air at the water’s surface or from air introduced into the aquarium by mechanical means. Since water is considerably more dense than air, absorption of oxygen is slow.

Turbulence increases both the amount and rate of oxygen absorption to a maximum of about 8-9%. Of the available oxygen in an aquatic system, about 80% is consumed by the fish and the remaining 20% is required by nitrifying bacteria to break down wastes. Trickle filters and airstones provide excellent aeration. Supersaturated dissolved oxygen levels are often reached in saltwater aquariums with high end filtration like trickle filters.

Live plants, although they produce oxygen as a by-product of photosynthesis, actually consume oxygen during the respiration phase. Further, plants generate waste that must be broken down by bacteria, a process also requiring oxygen. The more nutrients and food that enter the system, the higher the production of bacteria and the higher the biological oxygen demand.

Saltwater systems:
Dissolved oxygen can drop to lethal levels in saltwater systems with high salinity, high temperature, dirty filters, low or no lighting, and poor aeration or filtration. Warm Coral Reefs range from 4-6ppm oxygen. Many fish and invertebrates suffer when levels drop below 3.5ppm and few survive if dissolved oxygen drops below 2ppm. If levels drop below 4ppm:
• increase circulation with airstones
• add a protein skimmer
• upgrade to a trickle filter
• reduce fish population
• get a larger aquarium

Freshwater systems and ponds:
Dissolved oxygen can drop to lethal levels in freshwater systems with high temperature, dirty filters, low or no lighting, and poor aeration or filtration. In freshwater aquariums and ponds minimum levels should be 5ppm. Levels below 3.5ppm should be considered dangerous and can lead to fish loss. Ideal levels are 8+ppm and do not exceed 14ppm. At levels below 7ppm:
• increase circulation with airstones
• upgrade to a trickle filter
• reduce fish population
• get a larger aquarium

View Oxygen Test Kits and Monitors