The apparatus includes a carbon dioxide bottle (6) which supplies the water of an aquarium with controlled doses of carbon dioxide. The controller regulates the release valves to provide a controlled gas injection via a bubble monitor (2) and an injector (3). When the supply bottle is empty the controller automatically switches to a smaller capacity reserve bottle (7). A warning unit (31) operates to show that the system is on reserve. When the main bottle is replaced the system automatically switches back to main feed. The dosing of the carbon dioxide is controlled by the gas pressure and by a timer (4) and a pH monitor. The pH-value is kept to a set value which promotes plant growth and which is suitable for the particular marine life in the aquarium.
