Basically, the answer is yes, fans will remove your humidity. Opening the tank once a day to feed darts probably won't do anything significant, unless you keep it open for 1/2 hr. or something. The only thing I have found to work for me is to keep the tank completely sealed (or as much as you can), and only use the fans to circulate the air, not for air exchange. However, in a 4' tall terrarium, you should have enough natural air currents going on (convection). If you choose your orchids well, you shouldn't have any problems. Some really require a lot of air movement (mostly to dry out faster between waterings), and some like to stay moist and have really humid air that doesn't neccessarily move a lot. I haven't had any problems with my orchids. I suggest bulbophyllum/cirrhopetallum, and paphiopedilum. Phals are not good. They require a cooling period at night in order to bloom (more than would be good for darts), and I have found them to seem to need more air circulation as well.