Should my tank have voltage in the water?

So the other day I had a hangnail and placed my hand in the tank and noticed that I was being shocked. I checked the water with a meter and it was showing around 12 volts. I tested some of my pumps and heaters and noticed that my heaters and one of my pumps were throwing off some voltage. I purchased a new pump and a new heater installed them all today and noticed that my voltage is still around the same. I'm wondering if this is normal, do tanks have voltage just put into the water from the devices or should the voltage be at 0?

Also what is strange (to me at least maybe someone can explain it better) when my apex turns off my new heater the tank is showing 12v but when the heater kicks on the voltage drops to 8v.