This is the main reason I really dislike aquariums for caging. The only ventilation is through the top. However most people do like them because they make a much better visual display then cages specifically designed for snakes.
You might want to consider replacing one of your hide boxes with a plastic box filled with damp sphagnum moss. Put a hole in the side big enough for your snake to get in and out of, keep it moist in there at all times so he has a high humidity refuge.
A more expensive option may be to dump the aquarium and replace it with a visionarium from vision products. http://www.visionproducts.us/htm7/visionarium.html Without the screen top insert, get a solid top instead. You won't lose as much moisture when the ventilation is at the corners and it will still make a good visual display.