my cobra guppy had a split in his tail just come out of nowhere like that. was fine one day and then the next halfway down was split so i did a 50% water change, cleaned the filter in the tank water the opposite end of the tank to where the filter is placed (dunno why just seemed right) added waterlife acid buffer (humaquat). the tank then went really cloudy as expected and when it cleared up after a few hours the guppies tail went back to normal after a few days and its stayed that way ever since. if that helped an adult guppy it might help fry although i would not be able to tell if my fry have got split tails or not. so it could just be your fish if your tank is cycled properly.