I was afraid that might be the case. I already have 3 sides insulated with foam board, and a red heat lamp over the tank. The heat lamp was keeping the temps fine at night, until I had to raise it higher when I added the Plexiglas top. I think it's just too far away now. I may have to cut a hole in the plexiglass so I can lower it back down, but that may put me low on humidity again.
Unfortunately, I have the tank in an area off the living room, which is all open from kitchen, through living room and on through a hallway. Not sure a space heater is going to make much difference on such an open area, but I'll experiment. The blanket idea might get me through the colder nights though. I'll try that tonight and see how much it helps.
Thanks so much for the suggestions. I've been learning a lot reading this forum. So many helpful people here with so much great info! I appreciate you both taking the time to help!