i use flexwatt heat tape connected to a thermostat to heat my leo cages. i don't like using lights for heat because they dry out the air so much.

you want to create a temperature gradient in the enclosure. the cool side should be 78-80 degrees and the warm side should be 88-90 degrees. no night drop is necessary.

a 20 tall tank is also a little small for a trio of adults as they don't need the vertical space but rather ground space--something like a 20 gallon long would be better and also easier to heat via a light bulb.