Ventilation is essential for maintaining a healthy environment for your plants, as it helps to regulate temperature, humidity, and carbon dioxide levels. The amount of ventilation you need for your indoor garden will depend on the size of your space, the type of lighting, and the climate outside. Generally speaking, you want to exchange the air at least once every minute. For example, if you have a 4x4 tent with a volume of 64 cubic feet, you will need a fan that can move at least 64 cubic feet per minute (CFM) of air. However, this is just a general guideline, and you may need more or less ventilation depending on other factors. For example, if you use HID lights that produce a lot of heat, you may need more ventilation to keep the temperature down. If you live in a humid area, you may need more ventilation to prevent mold and mildew. The best way to determine how much ventilation you need is to monitor the temperature and humidity in your growing area using a hygrometer or a thermometer.