Your ? To why ph might be fluxing or going up/down:Because the ph down/up are wearing away , hints why organic versions if not last fir hours/small periods and aren’t recommended for use vs most synthetic versions. Last longer.
if you didn’t add any ph adjusters, could be your nutes has auto balancers ( like some are made for soil so it auto ph’s at 6.5-6.7 or hydro/coco 5.8-6ph)
if it’s bacteria or fungi doing such. For example if I apply seagreen before phing it. It will always make the ph to what u did when u applied seagreen and it will eat that salt/ph adjuster within minutes and I have failed a few times by having mot wanted ph and tried to correct it. You ain’t beating seagreen on that or salt
some fungi’s and bennies are more down/up ph and will stabilize your whole medium/water as such. Look at worm poop
