To keep my frogs warm I try to keep the light on at night, that way it should keep my tank warm during the colder nights and then I have the light of during the days when it is warmer. That helps prevent to much of a temperature change. If you need help keeping temps warm at night yuo could try using an electric blanket, I know it works when I'm cold at night it would probably do just as well for your frogs.
1.1.0 D. luecomelas
2.1 Felis domesticus