Carbon Dioxide Poisoning

Get ready for some heavy chemistry discussion as we understand the impact of too much or too less carbon dioxide in fish. It is a serious complication that can cost the lives of your fish.

The role of carbon dioxide·
  • Carbon dioxide is dissolved in water in its molecular gaseous state; only 10 % is in the form of carbonic acid H2CO3. These two forms of carbon dioxide together constitute what is termed free CO2. The ionic forms, i.e. fixed carbon dioxide, are represented by the bicarbonate and carbonate ions (HCO3- and CO32- respectively). Their presence is vital since they contribute to the buffering capacity of the water.

Toxic Action:
  • The toxic action of carbon dioxide is either direct or indirect. The indirect action of both free and bound CO2 is exerted on fish through its influence on water pH, especially where, as described earlier, the values rise to toxic levels. A direct adverse effect occurs when there is an excess or absence of free CO2. In waters of low oxygen content, such as where intensive biodegradation is taking place, or where fish are kept or transported in a high density, or when poorly aerated ground waters are used, free CO2 may reach harmful levels.

How it affects fish:
  • In such cases the diffusion of CO2 from the fish blood into the respiratory water is reduced, the blood CO2 rises and acidosis develops. If the rise in CO2 concentration is relatively slow (e.g. over 1 day), fish can adapt to the acidosis by increasing the bicarbonate concentration of the blood. Adapted fish can then suffer from alkalinosis if returned to water of low CO2 content.In water of low O2 and high CO2, where gaseous exchange at the respiratory surface is limited, the fish increase their ventilation rate, become restless, lose equilibrium, and may die. Twenty mg free CO2 per litre is considered the maximum permissible concentration for trout (higher concentrations can cause kidney problems) and 25 mg free CO2 per litre is the maximum for carp (if the acid capacity is 0.5 mmol per litre at a pH of up to 4.5). The sensitivity of fish to free carbon dioxide declines with increasing acid capacity of water.
  • However, the more frequent occurrence is a lack of free carbon dioxide in water. Carbon dioxide deficiency occurs when too much free CO2 is utilized for photosynthetic activity by the phytoplankton, or when the water used in thermal power plants is artificially softened or when water is aerated more vigorously than necessary with CO2 free air. Free carbon dioxide concentrations below 1 mg per litre affect the acid-base balance in the fish blood and tissues, and cause alkalosis. A lack of free carbon dioxide is particularly harmful to cyprinid fry when they pass from endogenous to exogenous nutrition. Cyprinid fry respire through their body surface and are unable to regulate their acid-base balance by gill respiration. A low partial pressure of free CO2 in water is conductive to a high CO2 diffusion rate from the body, leading to alkalosis and finally to death. If the fry of cyprinids suffer from free CO2 deficiency, they gather close to the water surface and show symptoms of suffocation even though the concentration of oxygen in the water is adequate (Taege, 1982).