Hi Hiss,
Yes, I think you are right to be concerned about alien species affecting our natiive fish stocks, Zander and catfish are a couple of forign introductions that imediately spring to mind. I think this is why, i recent years some cold water fish which was available as aqaurium and garden pond keepers have been removed and are no longer available for sale to the public, there is a long list but I recall Bitterling and shiners was some on the list.
I guess it takes just one fool to relese these fish and the problem will soon spread to other waters.
Even on the UK mainland fish being introduced from place to another can and does cause problems, apparently some fish that only exist in Scottish loch's are being driven to the brink of extinction by another fish (ruff or pope I think) which is also native but not indiginous to the loch, the problem is the introduced ones are feeding on the egg's of the other fish.
I'm no expert but I'm glad you brought this to light !
Good luck Tbird+