It has me puzzled a bit. I am having problems with the windows sweating. Live in a newer construction home, so the windows are pretty well insulated (doubled panes with the gas inside). My little weather station also tells me the inside temp and humidity. Just looked at it, 70 degrees inside and 50% relative humidity.
Primarily in the winter I heat with a pellet stove. I thought about the idea of running the furnace fan just to circulate the air around the home. Have it running now and will probably leave it on all night and see if that helps.
Oh, also, tank is a 200 gallon DD that is in the 'formal' living room. It evaporates exactly 2 gallons a day, everyday whether it is summer or winter. Not an expert by any means, but that doesn't seem like an excessive amount of evaporation.
What about you guys with large tanks and enclosed fish rooms? Do you guys have any moisture problems?