I have the same issue. I have 3 Hydor power heads of various models, sizes, and ages (3-5 years). All three of them are contributing ~20VAC to ground. I recently discovered the issue when I was messing with my tank while not wearing shoes. Apparently, the carpet in my basement does not provide enough isolation to prevent me from receiving a nice shock. I usually wear shoes when working on the tank, so I hadn't gotten hit before this. I pulled out the DMM and measured ~60VAC to ground! I isolated the problem to the three Hydor power heads, with each contributing ~20VAC. Once they are unplugged, all is well and there is not a significant potential in the tank. The fact that all three of mine have failed makes me think it is a significant issue.