I have known it for many years. And I think its only toxic if it gets near their gills or mouth or eyes, its mainly used for sores on fins and you only dap a tiny bit of iodine on it and then put it back into the water. When my fish get sores on their fins, I do that and they heal right up and my fish are healthy as a horse. But I heard it from some fish people and a pet store owner before.