Plain water and nothing else and at normal intervals and in normal amounts for the last 7-10 days is all that is needed for a "flush".
There is no need to drown your plants with gallons of water.
Ice cubes and extended darkness is ghetto science that has no botanical/scientific facts and is not something that that will magically do anything.
Just keep your plants as happy and healthy until the last moment, then chop.
Avoid anything that will cause stress to your plants right before harvest and after months of careful cultivation.