When you are growing cannabis plants, it is especially important to have plenty of oxygen getting to your plants.
A large amount of oxygen must be absorbed through the roots.
In order for the plant to accomplish this, the soil or planting medium has to have really good aeration. You can accomplish this by adding a good amount of perlite, gravel or sand to your soil or planting medium. Some people like to use rockwool as a planting medium because no matter how much you water it, it always has plenty of oxygen to get to the roots of the plant. It is also a smart idea to add food grade hydrogen peroxide to your water when watering your plants. The food grade hydrogen peroxide has an extra oxygen atom that will easily be broken down and can be used by your plants. Some people love to use oxygen plus plant food which actually has 25% hydrogen peroxide which allows your plant to get that extra oxygen that it needs. For more oxygen in the water, you can also use an air pump in your water to get it filled with oxygen before watering the plants or you can always put water in a container with a cap and shake it really good before using it to water the plants. Your cannabis plants will grow much better with more CO2 also. A good way to supply more CO2 to your plants is to visit them regularly. By breathing on the plants, you are exhaling CO2 naturally onto the plants. You can also use CO2 tanks, but this can get very expensive. A pretty good way to provide CO2 to small grow tents is to get a CO2 bag which is filled with mushrooms and ingredients that naturally release CO2 into your grow tent.