I changed my monstruous power supply fan (120 VCA, 65 cubic feet/min) for a smaller, less heating CPU fan (12 VCC, 32 cubic feet/min).
I noted the data concerning temperature and humidity fluctuation, and have an idea the difference cause by a powerful fan, a medium fan and no fan.
What surprised me though, is that I had believed a fan would lightly descrease temperature while greatly descreading humidity.
I was wrong, on the contrary, air circulation reduces heat by more or less 5°F, in both the case of a strong one and a medium one, but the airflow is very noticably increasing humidity! (and quite fast too, a 2% increase in the first 3 minutes since the installation of the small one inside.)
My hypothesis is that, since my terrarium is pretty much closed space, their is no cooler or warmer air being displaced from outside to inside or vice-versa, hence the small temperature drop. Also, the airflow is evaporating the water in the soil and trays (I placed a bowl of water in a corner precisely to increase humidity), but since there's little exchange between the ouside and inside, these evaporated mollecules of water have nowhere to go, and thus remain in suspension in the terrarium air, increasing ambient humidity.
(Btw, stronger fans create more air circulation, but while they col a little more, their engine creates more heat, so it's not much more effective than small one. The only use would be any effect of a stronger wind n the plants.)
These observations are valid for a closed, or very close to closed terrarium, I have yet to test on a more open one.