I have the same problem.
With two saltwater tanks in my back back room and a cement floor, I constantly stick my hands into the tanks with bare feet. Instant ground! I know, I'm stupid. If I wear sneakers or stand on a wooden stool, I never get shocked.
All saltwater tanks are going to have some leaking voltage due to all the submerged equipment, I believe this is normal. Once you touch the water you provide a current, you will get a slight stinging shock. Your fish may not normally feel it since there is no current flowing. Stick on a grounding probe and you will have constant current that may affect the fish.
I have GFCI breakers that never trip when I get shocked, so I am assuming the amount of current I am receiving is not enough to do any harm.