Whether or not you have an active or passive system, in the process of buying a fan and filter kit, there are some things to keep in mind. One of these things is the CFM, or cubic feet per meter, this is talking about how much airflow can be generated by the fan. It is easy to calculate, simply multiply the width and length of your room by the height; you should select a fan with a CFM that is larger than your grow space.
Look at your duct/fan sizing, many exhaust fans come in a large assortment of dimensions most commonly 4-inches or 8-inch duct fans, and 6-inch inline duct fans. The more space that you have the greater ventilation is required, thus a larger fan, which is why professional grow operations generally have an 8-inch fan; but smaller operations or home grows only need about a 6-inch fan.
Consider the duct length or filter type you are using to enhance your exhaust efficiency. It is essential for large grow operations, however, smaller operatives benefit from controlling this as well. If your tent is bending from the exhaust fan, this is a good sign, not a bad thing! This indicates a proper suction and that you have created a negative airflow. Also, ensure that your intake hole is larger than the exhaust hole for proper pull. Spaces for new air to come in (fans, door, windows, vents) should all be greater than the space where the air is being positioned out of the room by the exhaust fan.
Take into account the materials that you are using for your ducts, these must be lightweight, flexible, and most commonly are made of aluminum. Though this is a costly option- it will pay off, in the end, to install ducts for efficiency in ventilation.