Have you checked your tank for stray voltages?
You can do this with a voltmeter. Plug one end into the ground (bottom round one) on an outlet and the other in the water.
I saw that a guy on another forum had his fish stress out and die and his corals weren;t doing as good as his other tank. He did this and found that he had a stray voltage of 54 volts. It was caused by his heater and powerhead which were both relatively new.

If all else fails, you could buy a grounding probe and plug that into a GFCI outlet or buy a plug in GFCI to make the outlet that the tank is powered on safe along with the grounding probe.
I'm going to test my tank when I get home. If I find a stray voltage, I am going to get a grounding probe.