GFCI should be manditory piece of equipment. A grounding probe in conjunction with a GFCI is a good idea for added safety.

40volts is a lot. Some small voltage is expected and can simply be induced by properly working equipment being submerged in water. But seeing voltage at 12v or higher I would be very concerned myself as that is going to be a good sign that a piece of equipment is open or shorting. Some of our powerheads work on these 12v-24v low levels of DC voltage but that amperage on short can definitely lead to livestock problems.

Everything is on a GFCI and its only 30 volts. No one device shows very high so I don't think I have any equipment failing. Maybe I will do the whole process again to see exactly what voltage each device is giving off. Again most of the equipment is fairly new.