Problem with sensors when switching on water recirculation pump

Hello, I have recently purchased the 
"Gravity: Analog pH Sensor/Meter Kit V2 (Arduino & Raspberry Pi & micro:bit Compatible)" together with the module "Gravity: Analog TDS Sensor/ Meter for Arduino" and the isolator "Gravity: Analog Signal Isolator", Everything works perfect, I am very happy with the system until I plugged a fish tank motor to recirculate the water... then the pH dropped several tenths.... I unplug the motor and it stabilizes again... 
What could be happening? How can I solve the problem?  
Thank you very much...