Can I use the fan in my grow only when the light is on?? I mean, can I set it to turn on and off with the light in the same cycle, or should it always be on??
Keep it on all the time.
Airflow is great for plants for a variety of reasons.
When the light goes off, the humidity rises, with no fan this could quickly contribute to rot and generally reduces overall airflow to the plant.
Plants outside usually have breeze 24/7 too :)