PPM's do not diminish they increase.

Here's what's happennigng. You mix nutrients and get it to 1800. Great.

Now your plants use some nutrients and give off waste. Lets say almost the same.

However your plants use water way faster than nutes. Plants drink just one gallon and nutrient levels go way up. Top off with one gallon of ph'ed water and levels back out to about 1800.

This is why you top off with plain ph'ed water. Whenyou have added as much as your rez then change . So 5 gallon rez after I add 5 gallons plain ph'ed water. Time to change.