There are some times when you need to drop the water levels in your tank, for example, to change substrate.
You've spent potentially months or even years cultivating a nice layer of biofilm on the walls of your tank. So how do you stop the biofilm from drying out and dying off?
The simple solution is gladwrap/cling film/clingwrap. Simply lay a piece over each wall of your tank to keep in the moisture.
And when you refill the tank it will float to the surface for easy removal.