Bringing fish from Canada (or most any country) to the US requires following import procedures per 50 CFR 14:
http://www.ecfr.gov/cgi-bin/text-idx?rgn=div5;node=50:1.0.1.2.8
A USFWS import / export permit is only required for commercial imports (costs $100 and is good for a year).
Unfortunately, "commercial" has a stupid definition (especially in this case) - per 50 CFR 14:
Commercial means related to the offering for sale or resale, purchase, trade, barter, or the actual or intended transfer in the pursuit of gain or profit, of any item of wildlife and includes the use of any wildlife article as an exhibit for the purpose of soliciting sales, without regard to quantity or weight. There is a presumption that eight or more similar unused items are for commercial use. The Service or the importer/exporter/owner may rebut this presumption based upon the particular facts and circumstances of each case.
It really comes down to the individual inspector with whom you're dealing.
Either way, for a non-CITES import, you'll need to notify USFWS at least 48 hrs prior to coming, file an "eDec" (and/or bring a paper 3-177 ) and maybe pay live animal inspection fees ($164 if I recall).
Or you can tell the Customs person at the border that you bought some aquarium fish and hope they're cool.
Does Below Water ship to people in the US with Import / Export permits? I think Ted Judy did a shipment from him...
Matt