It would be faster to just do a fishless cycle because with fish in the tank you are going to have to do water changes to bring the ammonia down, which is the same as lowering the food supply for the bacteria.
Moving the old water into the tank only does one thing, dirties up your new tank. Also, one sponge would not have enough bacteria grown on it (unless there was very little filtration in the old tank, then the concentration of bacteria on the surface areas would be exponentially larger) to support fish.
Moving the old water into the tank only does one thing, dirties up your new tank. Also, one sponge would not have enough bacteria grown on it (unless there was very little filtration in the old tank, then the concentration of bacteria on the surface areas would be exponentially larger) to support fish.