AsNoriuanswered grow question 3y ago When fast goes yellow only ph issue , plus could overload of nutes. Calcium issue. But i would bet on wrong ph when tou transplanted, unless you ripped roots somehow, never did it, so cant tell, but loading anything like 5.5 or bellow would get such results quickly.
Some pens start to lie, have aquarium drops to compair readings after calibration.
I use 6.0-7.6 small range drops. Never lies.