Tony I would have a really good look at the grounding scheme on all of your tanks. Although I have not beefed up the ground on my tank yet
I have noticed that if I use say.. my dremmel... and put my forehead on the metal stand...there are stray currents. If your ground potential is not zero and you use this as a reference to measure the rest of your voltages...they will be out a bit.
I would run a very solid ground back to your panel if you can, and then really ground everything. Once this is done repeat the measurements to see if the problems persists. A simpler check would be to run an extension cord to the plug at your electrical panel (most have them) and repeat the same measurement using the ground on the extension cord. In all my years of doing electrical work the single biggest problem I have encountered is bad grounding.