Well you can catch only one of each Legendary Fish(And Legendary Fish II), everyone knows that right?
So, how come some people have 40+ Legendary Ponds?
If I'm not mistaken, you can only have 1 each of the Legendary fish, but you can have multiples of the Legendary II fish by doing the Qi Quest repeatedly.
It's possible to get infinite using the panning method, you can only no longer pan it when you've caught it and fulfilled the check for catching it (which removes it from the pool of available fish that the pan grabs from)
If I'm not mistaken, you can only have 1 each of the Legendary fish, but you can have multiples of the Legendary II fish by doing the Qi Quest repeatedly.