Do I have stray Voltage?

[PoonTang]

Well I am having troubles with the PE on my my acros and Milli's still. i was sure it was due to Red Bugs so I treated. I am seeing no improvement so on to the next possibility. Stray Voltage. I dont exactly know how to test for it so i put the neg probe on the ground terminal of my powerbar and the pos probe in the tank. I got a very erratic reading of 50 to 100 mVDC. Is this the correct way to test? and was that a bad reading to have?