Aquatics Unlimited: Articles: Lack of Oxygen
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Lack of Oxygen

for the week of 6/24/99

Oxygen is precious to all animal and plant life, including the fish in our aquariums and ponds. When it is in low supply, fish suffer, and may become ill or even die. Here's where oxygen goes:

Fish: fish and other aquatic animals breathe in oxygen and exhale CO2 to support their life processes.

Plants: most of us remember that plants produce oxygen by photosynthesis, but forget that they also use oxygen as well. While brightly lit, plants produce more oxygen than they consume, but at night they too are a drain on the available dissolved oxygen. Not only higher plants, but even lower forms like algae, consume oxygen.

Bacteria: the good bacteria and other microorganisms that degrade fish waste, ammonia and nitrite all require oxygen to survive and function. An aquarium or pond that needs a huge population of these due to overfeeding or overcrowding can easily become low in O2.

Chemicals: many chemical reactions, both natural and those caused by our additives, bind up or use oxygen. In particular, pond and aquarium medicines containing formalin can drastically reduce the amount of O2 available.

The total oxygen used by the first three groups is called the Biological Oxygen Demand (BOD) of a system, and the oxygen used by the last group is the Chemical Oxygen Demand (COD). If the total demand for oxygen exceeds its availability, problems will arise quickly. Fish are commonly seen to become lethargic, hanging near the surface or by an airstone or filter outlet, and eventually gasping.

A worst case scenario might be a tank or pond that is overcrowded, overfed, medicated, and full of plants after the light is turned off.


"Tip of the week" appeared regularly in 1999 and 2000.

Copyright © 1999 Aquatics Unlimited
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