In Canada we have no restrictions on importing aquarium fish because not many let free in the wild would survive a -40f cold snap! (With one exception. see:
http://aquarticles.com/articles/travel/ ... _fish.html ).
A popular thing to do is go to the Amazon and bring back wild caught fish, and of course we can bring fish across the US border.
We do have a few recent problems with introduced fish - Atlantic salmon escaping from fish farms on the Pacific coast, Russian snails let free from ships' ballast in the Great Lakes, and so on, but aquarium fish haven't caused a problem.
New Zealand has complicated quarantine regs. that I wrote about in my article about Gordon McKay in the People section of Aquarticles, and Bermuda has tough regs. that Craig Morfitt wrote about in the Travel section.
Howard.