If you look at evolutionary tree, it has been a while since frogs and fish became two different species(separated from common ancestor). Frog is Amphibian that does not have mating capability with a fish. Hence it should be assumed that even through mutation chances of tadpole becoming a fish is pretty much none. It’s pretty much evolutionary impossible for one species that has already experienced speciation to go back to its common ancestor(removing a mutation) and becoming integrated into a another species that is already existing.
