Winter Cooling Help for an amatueur - please!

[nicas83]

What's the right temperature to cool my Australian Green Tree Trog correctly? The enclosure is currently kept at around 15-19 degrees celsius (59 - 66 fahrenheit).

Do they normally take nightly water dips during cooling?

The reason I ask this, is that I read somewhere that if it's not cold enough, the frog won't cool properly and would still be using too much energy, but would slowly starve as it's still too cold to digest his food?

1st winter with him and I'm worried I'm doing this wrong. He is still taking dips in his water most nights and stopped eating his food 2-3 weeks ago. I'm tempted to just get the tank back to summer temperatures to ease my mind but thought I'd ask here first...any advice would be really appreciated!