I don't understand your question. If you have a fan, you're going to have air circulation regardless of any temperature differentials. The air is being moved physically, not thermally.
The fan doesn't produce anything, it just moves what's behind it forward and outward. The air it's "pushing" will be the same as the air it's "pulling" unless some evaporative cooling takes place, and then the air would have to go through or over something wet for that to occur, and the temps and RH already present have a play in that, too. Adequate circulation will "homogenize" the RH and temperature inside your terrarium.
Are you talking about blowing cool from elsewhere through a vent into your terrarium? This will cool it down some if that's what you're looking for, but if all you want it air circulation, then just put a fan inside your terrarium. Of course not sealing it is good, but doesn't really cut it IMO. You just totally lost me with this post. I'm not sure what you're talking about.