ideal fan placement

[Elmley]

I'm in the process of setting up a largish (100 gallon) vivarium for orchids. I'd be grateful if anyone had any thoughts about where the fan should go. I have a couple of small computer fans (I'm not sure that I would have to install both?). The idea is that the fans will both circulate air for the orchids' health and hopefully prevent the front pane from fogging. Is it best to have the fan on the back pane blowing towards the front pane, or to have the fan mounted upside down on the glass cover? I also intend having a small vent directly over the front glass - -say a 1" thin vent running the whole length of the front glass.
I'd be most grateful if anyone had any suggestions.