there is no spontaneous conception. Just because you can't say how they got in there doesn't mean that they got in there spontaneously.
If you have one fish (GSF or bullhead) it is possible that a heron regurgitated during a fly over. That is possible because it can be witnessed and therefore known for sure.
If you have more than 2 then it is unlikely that 2 herons regurgitated but it is pretty unlikely.
If you have more than 2 or more than a dozen then they got there because you stocked them (unintentionally of course), someone else bucket stocked them (very common although we would like to ignore the fact that this can happen), or they swam in.
Of course if you witness other portals of entry then we can be more confident about other possibilities.
otherwise disagreeing doesn't help.
The unwanted fish are innocent until proven guilty
