well.. tanks are tough with heat/humidity...

so what you want to do is trap the heat and moisture so what i would do is first ditch the heat lamp cause they suck all the humidity!! cut out a piece of cardboard, same size as the screen part of your lid and 1 or 2, 2 inch holes in the cardboard.. now completely tape down that piece of cardboard using aluminum/metal tape to your lid, dont leave any spaces for the heat/humidity to escape except for the vent holes you cut out! if you want you can even save the cut out holes and use them as vent hatches

at walmart you can find a cheap heat pad for sore muscles.. just throw that on top.. that should keep your ambient temp at 80 degrees and your humidity around 60..