why is gold fish called "gold fish" ? why not orange fish?

Answer 1

Probably the same reason greenhouses are called greenhouses and not glasshouses.
I've heard if you hold them up to the light they really do look golden but I wouldn't recommend it. (Don't try this at home kids)

Answer 2

Back when gold fish were first named, the color orange didn't exist. Once carrots were invented orange became a color.