I'd say with the others, the moral obligation is one of making sure the fish has a good home to go to when you run out of room. So it's about research, expertise, contacts- none of which should be a problem in your case. You wouldn't choose an unsuitable (unsellable) species or fail to shift it before the tank was actually creaking at its seams. IMO the moral problem would arise if you failed to provide for the fish; any profit made is immaterial from a moral point of view. Go for it.