This was my concern too. I have traded emails with one professor in our local University and I complained bitterly about the invasive 'problem'. He said if that was a problem we shouldn't be keeping any exotic fish at home.
The logic is simple; Singapore does not have any natural lakes or sizeable rivers, only small forest streams. 99% our indigeneous species are therefore of smaller size (1-3 inch) and even the Channa Striata are evolved to adapt in such small forest streams. The biggest problem came when some of these small forest streams were dammed/inundated to form reservoirs/lakes. Again most, if not all indigenous species can't survive in such enviroment because they were
NOT EVOLVED to adapt in such enviroment. So it is fair to say that their habitat were destroyed.
Now we have large volume of lentic water much like our tank. We have two choices; one is to keep the water empty. Another one is to introduce fishes that thrive in such lentic enviroment.
The professor says that as long as the exotic fishes in there does not find ways to the small forest streams which is typically few inches to 2-3 feet wide and couple inches to maybe 1.5 feet deep, what kind of 'problem' will there be? It is like after you build a new tank in your home and then someone tells you that you can only keep indigenous species. Does that make sense to you?
Yes we do have wild gigas.