TDG is the combined measurement of total dissolved gases in water related to atmospheric pressure. The TDG measurement utilises a pressure sensor attached to a membrane. The sum of the partial pressures is then measured by the pressure sensors, then related to total dissolved gas.
Total dissolved gas concentrations in water should not exceed to 110 percent. Anything above this level can be extremely harmful to aquatic life. Fish in waters containing excessive TDG may suffer from gas bubble disease meaning the bubbles can block the flow of blood through blood vessels causing death, though this is a very rare occurrence. External bubbles are also formed which can be seen on fish fins or other organic tissues. Gas bubble disease is also harmful to aquatic invertebrates, though at much higher concentrations than that which would be lethal to fish. Water quality standards for TDG were set at 110 percent.