You can test your tank to see if it has voltage in it with a simple voltmeter. Set it to read A/C current at a sensitive level (the meters have different scales-pick one that will read 0-25 or 0-50 volts). Touch one probe to the ground hole on an outlet (that's the big rounded one on a 3-prong outlet, or the larger slit on a 2-prong) and the other in your tank. Most ungrounded tanks have 3-10 volts in them. All tanks should be grounded. Stray voltage comes from heaters, powerheads, from your lights (salt creep can carry voltage from the hood to the water), and anything else that is electric. That voltage can cause lateral line disease in fish, or at least help it along. I don't know what effect it can have on inverts.
Sonny