The FDA has not thoroughly tested the zapper if any at all. I see time and time again that it works well but there is room for improvement. I have seen it cure tropical fish of both protozoal and fungal illness. These are illnesses that once established in an aquarium they will kill every fish in the tank. Using a zapper will stop the spread of these diseases and will cure the fish that are already infected.
What the FDA has told me is that no one has proven to them that the zapper does work and they do not have clinical proof that it is safe. They also state that because it is a device and is for use by humans, they have jurisdiction over it.