How do I regulate the temperature in my snake's tank?

I just recently bought a milk snake, and I started to use a Night Black light bulb at 100 watts in a 20 gallon long tank. I have two thermometers and one said 82 and the other said 90. I figured this was too hot so I went out to buy a different bulb. I bought a 50 watt bulb and it displayed 80 on one side and 85 on the other. My Apt is 72 at night and 77 in the day. Is this temperature range ok? What can I place in the tank that will change the gradient further? And is a heat pad better than a heat lamp for a milk snake?