Tank room is looking good Sanh. I'm actually impressed with your progress, these builds always go slower when it's not the first one.
I don't know about this, DC current is as inert as you can get. I would suspect the stray voltage is possibly coming from something else.
Also, saltwater is packed with ions. As these pass over metal (probe electrodes) electrons are exchanged. This alone is capable of producing a DC current. Try putting your multimeter in a 5 gallon bucket of NSW and swirl it around a bit. I bet you get some "stray" DC current in there