I use these TDS & PH Pen https://www.amazon.com/Water-Quality-Meter-Pancellent-Temperature/dp/B06XCMNFVQ/ref=as_sl_pc_tf_til?tag=_&linkCode=w00&linkId=&creativeASIN=B06XCMNFVQ I calibrated it a few weeks ago, I'm about to do it again tomorrow.
Will this do fine? Water Quality Test Meter Pancellent TDS PH EC Temperature 4 in 1 Set https://www.amazon.com/dp/B06XCMNFVQ/ref=cm_sw_r_cp_apa_qdc9Bb0HB8EZX