Ugh, I'm so sorry to hear this. Unfortunately, this kind of thing can happen from time to time. In answer to your question about the other fish, very likely they WERE affected, just not to the degree the tang was. As a "sensitive" fish, the tang probably felt it more acutely due to the thin skin they have.
Now, let's talk problem solving. Obviously, something in your tank has eroded enough to expose a wire. Salt water is highly corrosive, and this kind of thing is almost fated to occur to some of us eventually. With your power out, look at the obvious culprits first. Check your heater - does it look normal? How about your return pump? The saving grace is that there actually aren't many things that involve honest-to-goodness "cords in the water" for our tanks -- a heater, protein skimmer, return pump, and powerheads are usually the go-to guys for stray current. Start with the item that's oldest (or, and let's be honest, the chinese knock-off items).
Let me be clear. All of our tanks - every single one of them - has a small amount of "stray voltage" that can be measured with a multimeter. I don't have the technical experience to explain it properly, but a short internet search will pull up any number of articles explaining how even stray voltage from your lights - outside of the tank completely - can penetrate the tank and cause a readable signal.
It's not the voltage that's the problem, however...it's the current. It's how a bird on a high-voltage wire can sit there on a 10,000 volt line with no ill effects whatsoever. The voltage doesn't mean a thing, unless said bird was to make contact...say with another wire. Suddenly, the bird is acting as a conductor, and the
current of electricity passing through his body will almost certainly immediately cook the bird to a crisp.
Same goes for our tanks. In fact, there are more and more arguments to be found that seem to indicate that our grounding probes we use are actually
harmful in these situations. A bare hot wire will find the shortest path to ground....and if the grounding probe is WAY ON THE OTHER SIDE OF THE TANK, then there is a long path for current to pass in order to ground out!! GFCI's don't always trip in these cases either; the current may be high enough to be harmful for our animals, but not high enough to trip the interrupter. That's assuming the GFCI is even wired correctly...an unfortunate possibility.
There are plenty of ways to test your tank for stray "voltage" using a multimeter, and you can google that easily enough. I would strongly recommend you START by doing a visual inspection of all the items going into your tank. If you see something obvious, leave that unplugged and test your tank for voltage using the multimeter. Be aware that you will see SOME voltage (it's the nature of the beast) especially if you're using digital multimeters, which are much more sensitive than the old style swing needle varieties.
If, on a visual inspection, you don't see anything immediately wrong with any equipment, you will have to start plugging in and testing. This is best to do with a partner. Have your husband reading the voltmeter (this takes both hands) and you start plugging in one item at a time. Have him read out the meter. Once you see something spike, unplug that item and continue the test. There may be more than one faulty piece of equipment after all...in fact, if you don't use a grounding probe, it's very possible that your current is generating BETWEEN one faulty pump to another! Continue this until everything is tested, and obviously, throw out and replace anything that is causing a large spike.
One more thing to point out. I'm sure you know it, but I'm reminding some of the newer folks. NEVER, EVER, "test" for voltage with your hand!! You may have gotten lucky and just got a mild shock, but this is electricity, water, and human life we're talking about. Don't be foolish about it. Inspect with the good old Mark 1 Eyeball. Test with a meter. Don't go dipping your fingertips to see for tingly feelings.
