A few things that also can affect humidity. Water bowl size....too big of a bowl will have more surface area and create more evaporation.
Air flow near the tub? If theres little air movement then will take more time to evaporate. Also wheres the water bowl placed? If it is placed on a warm end it will definately evaporate more water.

Not much you can do if the room the tub is in is Humid. Get that under control then the tub should follow.