Humidity

Some sphagnum moss spread around will help too. I also knew someone who bough a small square Tupperware thing, filled it with damp moss and punched some holes it it. It kept his substrate from being wet, and was enough to raise his humidity :) Good luck!

The single biggest issue that you have with low humidity is your enclosure. Fish cages are notorious for not holding humidity well.
If you're really stuck on keeping your fish tank, try this:
Purchase a 30 gallon fish tank and cut around the top of the tank with a glass cutter so that the height of the tank is about 7 inches.
Remove the mesh lid and purchase a 1" thick sheet of plexi.
Cut it to size, with some extra for a good fit up top.
Next, drill 100 or so holes (for air circulation) around the entire tank with a 1/2" drill bit about an inch or two from the top.
Newspaper/paper towel as a substrate.
Large water bowl large enough to easily soak in at the cool side.
Heat tape/thermostat underneath set to 93.
You will NEVER have a humidity issue again.
Good luck.