The technical term for low oxygen concentration in water, which finally leads to fish mortalities, is hypoxia.  This is the condition where the oxygen dissolved in the water is inadequate for the fish to be able to breathe normally, resulting in stress and ultimately death.  Different species vary enormously in their ability to take up oxygen from water, and also in their tolerance of such low levels, and even within a species the smaller fish tend to be more efficient oxygen harvesters than larger fish.
 
The environment of an earth pond is generally based on culturing algae to stimulate the food web, providing natural feed and thereby reducing the feeding costs of the fish.  Algae photosynthesise by day, giving off abundant oxygen into the water to the delight of the fish and other organisms in the pond.  Unfortunately, the algae cannot photosynthesise at night and rather switch to respiration, competing with the fish and other aerobic organisms for oxygen.  Under nocturnal conditions the oxygen levels in a pond therefore plummet, requiring the use of manual aeration to avoid the dissolved oxygen levels from dropping too low to sustain the fish.  This situation is aggravated by several consecutive days of cloudy weather, which reduces photosynthetic oxygen production by day, causing the night time concentrations to fall even lower.  Yet another compounding factor is water temperature in that warm water holds less oxygen at saturation than colder water, so summer waters are naturally lower in oxygen than the same system in winter.  Cruelly, the metabolism of our fish is higher in warm than cool water, further increasing the demand for this critical element that is in short supply under these conditions.
 
In recirculating systems the fish are stocked at high densities and oxygen levels are sustained by means of water flow and aeration or oxygenation.  Should these mechanical methods of life-support stop functioning the fish will quickly become stressed and display symptoms associated with hypoxia.
 
When the concentration of dissolved oxygen falls below the level required by the fish for comfort they will typically respond in predictable ways, displaying visible symptoms including gasping at the surface, congregating at the water inlet, piping (sucking in surface water) and rapid, exaggerated gill movements.  If the situation is not corrected they ultimately they tend to die with their gills flared and mouths wide open.
 
In ponds, cages or recirculating systems, the amount of oxygen in the water will be reduced if the fish are stocked at a density that is too high for that environment or if the culture tank has high levels of suspended organic particles.  These particles are colonised by bacteria and other microorganisms which require oxygen as they breakdown these tiny specks, thereby reducing the amount of oxygen available for the fish.  For most fish species the minimum level that fish are happy with is around 5mg/?.  At this oxygen concentration the fish can digest their feed efficiently and grow well.
 
It is therefore important to observe a few basic rules in order to prevent hypoxia occurring in our culture tanks.  These rules are simple and include the following:

• never overstock the environment
• avoid the temperature increasing to levels that exceed the optimal for the species and the design of the system
• keep the amount of organic material in the water as low as is practical
• ensure there is adequate aeration to maintain the required dissolved oxygen levels.