Heya Hebron,
you missed a couple things in the design of the hawk tank. You use the air pump, sealed in a box with a filter, so it sucks in clean air through the filter. I prefer filter discs from www.fungi.com ... they are cheap and work great. Next, you also need a couple of small holes, about 3/8" diameter drilled in the top plexi glass lid. Glue or tape a filter disc over these as well. AS your pushing fresh air into the tank, it is also pushing out the expended air, CO2, through the filter, and any cracks in the seal.
Also with the filtered hole on top, it allows a small amount of air exchange there as well.
Personally, I always had great success with this method. Have your air pump on a timer, not having to fan it, which increases the chances of more contamination infecting your casings every time you open the lid.
Using perlite is key. Use about 2" on bottom, with about 1" to 1 1/2" of bottled mnt spring water, also use about 20 tablespoons of hydrogen peroxide every couple of weeks to help the water from going stagnant.
With this setup, the humidity always stayed between 82%-92% , perfect for casings.
ONe last important note, dont use anything less then 3/8" thick plexi glass, drilled to a wood frame, or it will warp.
-peace- www.thehawkseye.com