I'm unfamiliar with setup but u can put an amp probe around a filter water line and see if ur geting amps. If so then ur wall socket may not be grounded well. It sounds strange I know.
Glad it worked but that solution sounds weird, perhaps from a not well thought out design on the device. Well at least it's only $18.After looking at schematics on your link I had an idea. I connected copper wire from aquarium water to ph interface gnd and it worked.
There are still some ripples around 0.05 ph but that can be fixed by averaging readings.
I am using this http:// http://www.sparkyswidgets.com/product/miniph/
Thanks to all