How I control my medium ph from rising is through chemistry or you can just keep flushing all the time like did in the past or you can control your ph....
PH stands for Potential Hydrogen and is a measurement of Hydrogen and Hydroxide ions and is measure from 0 to 14 and 7.0 being neutral if I remember correctly at 71 or 78 degrees F,this will raise and lower with temps
When your mediums PH is below 7.0 it contains MORE Hydrogen ions than Hydroxide ions. and vise versa alkaline state has mor Hydroxide ions(7.0 or higher)
Now these interact with each other to perform a balancing act that determines the PH in your water,growing medium and nutrients.
So these Hydrogen and Hydroxide ions are in the elements that make up our macro and micro nutrients and are classified in two groups Cations and Anions
Cations contain more Hydrogen ions
Anions contain more Hydroxide ions
Our growing medium is dramatically affected by these cations and anions contained in the micro,macro and secondary nutrients.REMEMBER this because this is very important when selecting nutrients for your medium.
Micro,macro and secondary nutrients that are cations are urea and ammonium(both forms of nitrogen),potassium,calcium,magnesium,iron,zinc,manganese,copper,and cobalt
and the elements that are anions are nitrates(another form of nitrogen),phosphates(phosphorus)sulfates,carbonates,bicarbonates
This covers most Cations and Anions
So get your books or Pc and research back to basics NPK.If your PH is rising in your medium,use nutrients or make nutrients that have more Cations(hydrogen ions) to bring your PH down.
Good Luck and Happy Growing