Thanks, all. Looks like I will need to buy a volt meter. I suspect my heater as a source, but don't know for sure if there's any current in the tank. When I saw more dead fish yesterday morning, I put my hand in the water (first thing I do when I think something is going on with my tanks) and felt like there was electric current in the water. But I could be making it up. Anyhow, I unplugged the heater and no dead fish today.
So if fish doesn't feel the current in the water because they are not grounded, it still impacts fish enough to kill them? If the current kill my fish, wouldn't more fish be dead rather than just a few?