With open lamps over your water it is normal to have stray voltage on your meter. What you do not want is current. I have seen voltage as high as 48 volts in an aquarium and never felt a tingle. I have had only two volts and it knock the heck out of me, it is current not voltage that shocks. Your best protection is a GFCI receptacle and grounding probe. Other wise your GFCI will not work because your tank is insulated. I had an old post here on MAAST about stray voltage