First off, ditch the fish tank and get a PVC cage. That will alleviate 90% of your problems. Then use a big water dish with Reptichips and you will have issues with how to lower your humidity lol.

But pretty much what everyone else said about covering the top. The key is you need to limit the airflow in and out enough to let a large water source aka a big water bowl with lots of surface area build humidity. If you look at PVC cages, you will notice one thing they all have, very little air holes. In your tank, cover the top and leave like an inch of either side which will provide plenty of airflow as the cool side the air comes in, goes across the cage and the hot side will lift the air out.