Being abusive to people is not the way to get helpful replies. You must be quite young.
I'm a new grower and I even know the answer to this question. You dump the res so you can sterilize it and ensure a proper mixture. You're reading ppm but that doesn't tell you what's in the water and in what quantities. So, you fill the res up with a fresh mixture that you
do know the quantities of. From there, you put straight water in one day and then a half strength nute mixture in the next. What's a half strength nute mixture? Well, you can either read the back of the bottle and divide the ml/g in half or you can figure out what your ppm/g is and then divide that in half. In other words, if your nutes call for 10ml/g you would use 5ml/g. If you figure that your 18g res with a ppm of 1600 has 88.88 ppm per gallon you would set it 44.44 ppm per gallon. That would be half of full strength, more commonly called "half strength".
Surely a master such as yourself already knew this, though.