Basically up until the mid 90's it was a free-for-all, with only certain species such as piranha and anything venomous being banned. Then someone decided that it would be better [from an environmental perspective] to restrict imports to species that are already in the country and haven't caused any problems, we have koi, rudd and bullhead catfish established in many rivers displacing the native species, which is part of the reason behind the restrictions. Kind of ironic given that trout have had equally, if not worse, consequences for the native species but are treated in a completely different light because they're "sport fish". A survey of current species was done and the list drawn up, there have been a few additions where they had missed things that were obviously here, but for the most part very few new species have been added to the list since it's creation. It is very difficult, and expensive, to do so as peer-reviewed scientific papers must be used to prove that the proposed species pose no risk.
Aside from the fish laws it's a great place to live! So on with the thread...