Ill give my 2 cents
I have about a 300gal tank and it (so far) seems to be between 70 - 85 with just two 150watt bulbs(and flourecents but there pretty irrelevant to heat). But i had the same problem only with heat and humidity. I had screened sides and a screen top, i used the shrink wrap stuff for your windows for the side screens so you can still see through them and amazingly that got my tank temps up to about 75-80 and it raised my humidity and kept it around 30% which was an improvment. So i had my heat now i had to figure out humidity, which like what was said the heat lamps dry out the tank.
So i took some old cermamic tiles i had and broke them in pieces and layered them around all the heat lamps on the top screen and amazingly it raised and stabled the humidity around 50% and on top of that i reach a scorching 85-90 so after i mist (waiting for new nozzles so i do it the old fashion way) my tank maintains about 70-80% for atleast 5 hours with both heat lamps on.
So im wondering if a 150watt is maybe too much? Also all your decor like wood and plants and soil hold moisture. Just putting my plants in can raise my humidity 20%.
I also move my humidity meter around constantly cause whats 50% in one place (for instance under the heat lamp) could be 90% down in the lower parts of the tank. I had my heat lamp over the pond to and i never noticed an increase in humidity in the tank, but i fill my sump with 2gallons a day easily. so i moved that.
Try to remove your heat lamp put your towel or a piece of wood over whats left of the screen up top, mist your tank, see what your tank gets to in humidity, also get a digital humidity thing with min/max helps alot cause its almost instant readings.
Sorry for the long post hope it helps.
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