Plants drinking water is a good thing. When they stop drinking is when you have a problem.

The plant drinks water and some PPM's but much more water. That makes your nutes ppm go up. Top off with PH'ed water untill you have replaced your rez. 5 gallon rez 5 gallons top up and it's time to change.

Also about your ppm's. Your plants use nutes but they also give off waste that show up as ppm's. So the ppm's don't change a great deal as long as the rez stays topped up.