The humidity inside the tank will be like the humidity outside. I know you can get a dehumidifier from Sears, Lowes, or someplace like that. I wouldn't imagine you could make your own, but i've never thought about it. As long as the humidity isnt very high for extended periods of time you should be just fine. If you are concerned about it, you could try moving more air over and around the enclosure to keep it from sitting and getting stagnant.