This refers to performing a partial water change (PWC) on a aquarium tank.
Typically done to remove dirt or waste, to lower or dilute a toxin like nitrogen (ammonia, nitrite or nitrate) or to simulate a flood to trigger breeding in some animals.
The usual amount of a water change is 25%, with 50% being done in emergencies, it is rare that a total water change is need or performed due to the high risk of harming your aquatic animals or plants.
It is important to match the temperature of the replacement water to the tank so as not to stress the animals or cause internal organ damage as fish in particular are sensitive to their water temperature as it governs their organs rate of metabolism. For example if a tropical fish used to a stable temperature of 25°C (77°F) is exposed to temperatures of below ~15°C (59°F) for a short length of it may suffer brain or other organ damage.
- See also pH Shock