[ecichlid]

I'll save you a lot of thinking and trouble. Just purchase the 60mm fans that member hydrophyte sells. They are quiet.

Humidity dropping would only be a problem if the fan is drawing in outside air. I should have specified that I'm talking about a solid glass lid with a fan mounted to its underside.

Unless your viv is sealed, this is an incorrect assumption. If there is air being circulated, some of the air will leave the viv thru seams and be replaced by air from outside of the viv.

Personally, I think gas exchange is a good thing for your vivarium. Why not have some venting?