I don't know about new plants causing it, but fog is a bad thing. It means you have poor air circulation and ventilation. You want high humidity, but without ventilation or circulation you will get mold.

-DH

well it just doesnt seem like it has any humidity in it and i dont have a humidity tester thingy

The only way to really know for sure is to get a 'humidity tester thingy' =) (hygrometer). =) They're not expensive, and they'll take the guesswork out of it. Some condensation is good, but too much is bad, because they you will get mold. Also, I've found pings to rot in the high humidity levels that neps love. New plants would not cause a loss of humidity. Opening the tank to put the new plants in, however, might.