Corleone
Fish Herder
I highly doubt that fish tank lighting emits radiation or mercury.
Unless the bulb breaks, the mercury is contained. As for radiation, everything emits radiation, including the fish themselves. Electronics are negligibly above background, and the lighting primarily produces radiation in the visible and infrared spectrum.
As for dogs and cats, as mentioned: Virtually all domestic animals are hybrids to some extent, bred by humans from multiple wild stocks. Because of that, without us, nearly all our domesticated stock would die off in short order. Many breeds of dog and cat are physically unfit to hunt, and many others have had the hunting instinct bred out of them (intentionally or unintentionally). For that matter, a good deal of highly bred fish that aren't generally considered hybrids were probably crossed at some point, as it's common practice in selective breeding.