Not sure if it's a fact or not, but I read it on the internet - so it must be true... right?
The actual thing I ran into a bit ago was a quote from a doctor that said through normal sweating and breathing, the average person pumps out 1 liter of water a day into the air. So doing the math and rounding up a bit, that's where I got the 2 gal
With my 46g, I lose about 2 gallons/week in evaporation - so from a humidity standpoint my tank's evaporation is the same as adding another person in the house. I also have a digital humidity thingy (technical term) in the house, and I haven't noticed any increase in average humidity levels since I've added the tank.