He's pretty big now, so if you haven't seen him nip at the fish I wouldn't be too worried. Contrary to the stereotypes, its just as common for fish to harm octopus as vice versa; although if yours was being nipped at by the fish, I don't think he would show off like that.
If you were to take it to a LFS they would almost certainly kill it. You have a very happy accidental octopus and you can consider yourself lucky
If you are worried about the fish and want to keep trying to supplement its diet, you can get fiddler crabs for reasonable prices at
www.aquaculturestore.com. I don't know if they ship to Canadia, though!
It does look a lot like
Octopus briareus, particularly the "web" at the base of its arms, but I don't think that's exclusively diagnostic. If your LFS is correct that it came from the Pacific then it is certainly not
O. briareus. The question is whether or not the store really knows where their stuff comes from. Either way, its no dwarf!
Thanks again for sharing. Beautiful little guy.
Dan