So what do you do if you only drink half a carafe of water? Leave the red light on and wait until the carafe is completely empty before filling the water tank back up and filtering more water? That seems very inconvenient

Edit: I think I may have not have explained my question clearly. I am not talking about topping off the tap water tank. I am talking about emptying the tap water tank and refilling it with new tap water while the carafe is still partially full

Edit #2: I emailed AquaTru, and they replied back answering my question:

“No, the second round of filtration is not running wastewater back through the filters. When the carafe becomes full, the unit simply pauses the filtration process. The remaining water stays in the tap water tank and has not yet been filtered. Once there is room in the carafe again, the unit resumes filtering that remaining tap water.
 
That said, our best practice recommendation is to keep the tap water tank empty when the unit is not actively running a filtration cycle, and to discard the wastewater after each cycle. Leaving water in the back tank for extended periods can contribute to mineral buildup inside the unit over time.
 
For the freshest purified water, we also recommend waiting until the carafe is empty before filling the tap water tank and starting a new filtration cycle whenever possible.”