Ro water and good Ph chems will help keep it down. There are some buffers in the nutrients to help stabilize Ph but in hydroponics it's normal to check and balance it twice a day. Most don't have as much of a problem because they use larger rez so there is more of everything to keep the ph in check but in your small waterfarm there is not as much water or nutrients so there are less buffers.
So in a waterfarm it;s normal to adjust the Ph twice a day.
Welcome to hydro.
If you go cheap you grow cheap. - Me