When I start a new res with a new crop usually what happens too me is that my solution which will have a PPM of let's say 1000 and a PH of 6, the PH will increase (I buy more PH down than up) and my PPM's will increase.
Once my tank is set up (I use a RDWC drip with Hydroton), I simply add more water to keep my PPM's in check (no more nutes), and adjust PH accordingly, mostly down, but sometimes I have to bring it up.
That's the name of the game in Hydro, it's adjusting PH, which is extremely important.
No big deal, but it will fluctuate and should be adjusted daily.