They have to be in luggage, not carry on (liquid restrictions). If you bought them, a bill is needed, as with anything else you buy. If you don't declare them, and get caught - not good. But if you are open, they are humanely packed and the numbers are small, we can do it.
They can't be on the CITES list of endangered species, and there is a listing of fish that need veterinary inspection because they carry diseases. A lot of gouramis, for example, can carry Iridoviruses. You have to do your homework.
A lot of really good fish are hard to find in Canada. So it can be worthwhile.