Biological CO2 supplying systems are a great and low-cost way to supply smaller aquaria with CO2. CO2 is produced by yeast fermenting a sugary solution. This solution is needed for CO2 production, and it needs to be replaced at regular intervals so the yeast can always draw on ample amounts of sugar for producing sufficient amounts of CO2 for your plants.
In larger aquaria, biological CO2 supplying systems are rather complicated to run as the fermenters need to be quite large.
In addition, it is almost impossible to adjust a biological CO2 supplying system finely. In contrast to this, a pressurised CO2 supplying system is far more convenient and can be adjusted much more precisely to the needs of the plants and the animals in your tank.