heat question...

[iBaman]

So, while I'm building Pajamas' enclosure, he's living in a 125 gal tank. It's got overhead lighting, which is nice...but I"m having a problem with heat (go figure, right?) I've got an 18"x18" reptitherm heat mat in there. The hot side is around 90, but the cool side hovers around 79-80. As soon as I turn the overhead lights off, it plummets to about 75-76...is this still ok, or should I get another form of heat? Also, can I put tin foil around his enclosure to keep heat in? Would I put it inside or out? (haha, usually I have humidity problems, not heat!) Btw, i'm using the hydrofarm thermostat with the heat mat.