My setup is a growrack that is about 95% enclosed in mylar and clear cellophane, with a couple vents up top(the 5% uncovered) for the warmest air to exhaust. I have a humidifier on the outside, with its humidistat inside. It draws fresh, dry air from outside the terrarium, and I have it routed to blow the cool, humid air inside -supplying both humidity and fresh air. I live in such a dry climate that my humidifier has to run quite a bit. Inside the growrack, I have two 4" fans that run 24/7. It doesn't affect my humidity, as my humidifier will automatically compensate. I don't have any mold, mildew, fungi, or disease problems that I know of. I attribute most of this to the fresh air incoming through the humidifier, and through the constant air circulation.
It sounds like you setup allows humidity to escape easily, and in that scenario, I'd just run the fan at night to help prevent the plants from drying out too much in the day - as long as it doesn't allow the temp to get too high in the daytime also.
As far as the red leaves with spots, I've had a couple neps do this in very bright light. Not sure if that's what may be causing it in your case though.