To chime in from a Toronto perspective, my go to store for fish is Reef Boutique in Toronto near Yorkdale mall
Without a doubt, Colin has the best quality fish at reasonable prices. He quarantines and stands behind everything that he sells. I have never (NEVER) lost a fish that I have bought from him.
Highly recommended
Another fish place in the GTA that I have started to shop at more is Discount Dragon Aquarium - 888 Dundas St E in Mississauga - the SW dude there (Andrew) seems to be accessing a wider variety of species and high end clowns. He apparently will source any fish or invert and really knows his stuff. This is a store that has raised it's reputation in the last year so if you haven't been there in a while, it's worth the drive
As for corals, I do like Coral Reef Shop in Burlington. They also have a pretty good fish selection, but they always seem to please for their wide selection of corals, clams. Both SPS and LPS
Also Canada Corals is a great corals shop near Pearson airport and they gave great quality and selection. In my opinion though, their prices are a little higher than some.
I sadly, haven't visited some of the other shops in SW Ontario or Hamilton so I can't comment on them