strange voltage in the tank

hi guys, i've got a strange problem in my tanks. if i submerge an electrode of the voltmeter in the water and the second one tuch the neutral one or the earth i would get some voltage on my display. whatever equipment i put in the water and switch it on i will have something like 15-17v on the display of the voltmeter. for examplε: with external filter 18w i get 15v, with thermostat 150w i get 18v, air pump 3v, lighting 72w i get 20v. this moment i have about 36v potential in my water. i have two tanks in different places, nothing to do one with other, and both of them have the same problem. i ask some others hobbist to check this isue and they have the same indications. what is going on, i am engineer mechanical but i can not give the awnser to this question.
nick

mvigor said:

Is your home and the outlets in question properly grounded? How much salt have you added to those tanks?

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i can not say if the building i live is grounded properly, in three different places in the town i asked there are the same problems. have you made the same measure in your tanks? try it please.
i do not add any salt in my water ever.
by the way, the plants you have in your tanks are nature or synthetic?
very nice composition especially in the living room tank, i liked it very much!

best regards
nick