The Ph & PPm will change as the plant uses up different nutrients at different stages of the life cycle. The plant will put back some used up nutrients as well and that changes your PPM. If you are doing a change every 7 day's then you should just add RO adjusted water around day 4 of 7 and bring the res back to normal. You should be adding back nutrients. I only change mine every 10 day's and I flush for 2 day's between changes too.

You should expect that, what are you using for Ph up & down?

Your plants drinking more water is a good thing, mine drink up to a gallon or more each day sometimes. You did not say what the temperature was?