Even at room temperature, the humidity in a fish room tightly packed with tanks can be significant. Where cool air enters the room through a crack, the moisture condenses on nearby walls and the ceiling, creating ideal conditions for mold. A sponge dipped in a solution of bleach and water can retard the mold, but moisture can enter the walls, and thence the house, and ultimately the roof. One breeder we know had to replace the roof of his house.
One way to combat moisture is to exhaust it out of the room. An industrial bathroom fan attached to a humistat (similar to a thermostat, only it measures moisture) provides a means of controlling the humidity level automatically.
An even better solution is a de-humidifier. The de-humidifier will give you precise control over the amount of moisture in the room. In the winter it will help heat the room in colder months, making it more cost effective than an exhaust fan, which pours warm air outside.
If you are building the room from scratch, cover the studs with sheets of plastic (poly) and seal the edges of the plastic with special tape you can buy at the hardware store. Seal around power outlets as well. Paint the walls with three or four coats of a latex paint. Cover all holes in the wall.
You can reduce the amount of moisture in the air by covering your tanks. However, if you have a lot of tanks, the covers will get in the way of cleaning and require cleaning themselves. One solution that works for us is to lay thick poly sheets over the tank, stapled to 1×3’s:
 Concrete Floor: Paint or Not?
Sealing a concrete floor and painting it prevents moisture from sinking through the cement. A painted floor is easier to keep clean. The best time to do it is before you move in the fish as the fumes may be toxic. You will need to put at least two coats of enamel on the floor to seal it. Painting the floor reduces dampness and helps keep the cold from seeping up from the floor in the winter.