That is definitely a big task to figure out.. do you have a photo of the tank to post in this message thread?
I could be wrong, but i think you'll have to empty (or nearly empty) the 117g to move it - otherwise you risk structural damage to the glass and/or seals. If the tank were emptied of water, would you still have to take the heater out or could it stay where it is?
Would you have water hose to run up to your granny flat thru a window or open door and into the tank? Then you could fill the tank with tap water, plug everything in & start it all running and treat the water.
As far as the fish being in water too cold - would you have any sort of clean temporary container that you could put them in? If so, you could put it in a spot that would stay warm (like near a stove, etc.) & fill it with treated water heated to about 26-28c? If necessary, you could do 25% water changes with treated water heated to say 30c whenever the temp started getting low to bring it up slowly and keep it in their comfort range while getting the big tank ready.
Then when the big tank is filled with treated water & everything's running & the big tank has a water temp close to the temporary container's water - you could transfer the fish into the big tank.
Not sure if this plan works or not, but i hope it's at least a start. Best of luck with the move.
