There are two basic requirements for plants to grow through photosynthesis; nutrients and light. The most important nutrient for plant growth is carbon. Since carbon dioxide (CO2) is abundant in the atmosphere, plants simply draw their carbon directly from CO2 in order to photosynthesize and grow. In an aquarium, CO2 levels are naturally very low and without any supplementation, submerged aquatic plants will not be able to grow properly. When CO2 is a limiting factor, plants cannot fully utilize all the available light for photosynthesis. This can allow algae to grow and compete with the plants for nutrients, further stunting plant growth. By introducing CO2 into the aquarium, plants can achieve healthy growth faster and therefore outcompete algae.
The introduction of carbon into an aquarium is typically achieved by using a pressurized gas tank controlled by a regulator, solenoid, and needle valve. Pressurized CO2 is the best way to go for the more advanced hobbyists or for those keeping larger aquariums, 20 gallons, or above.
The CO2 regulator is one of the most important aspects of any CO2 system. The pressure in a gas cylinder can be over 700 psi which is way too high for safe usage in any application. The regulator plays the crucial role of allowing just enough CO2 to dispense safely through the airline tubing, through the diffuser, and into your aquarium.