Plug the black input into COM and the red into VΩMA. Turn the multimeter on to the V~ setting. If you have different numbers, do the 200 option. Plug the black lead into the ground part (round hole) of a socket on your power strip or wall outlet and then put the red lead into the water of your tank. The number outputted will be the voltage. Try this with your grounding probe plugged in and with it unplugged.