If you want to filet fish the Japanese way, you need a deba. It's a hefty knife with lots of steel to withstand fish bones. The single bevel is designed to get the edge closer to the action and also to keep you from dragging the surface of the blade over all your beautiful fish meat.
About the shape
Deba means “short fat tooth” and this describes the shape of the knife. This is the first knife a sushi chef would use when preparing fish. It is used to fillet fish and butcher the boneless meat. The Deba is a traditional Japanese blade that is only sharpened on one side. This is called “single bevel.” Don’t let the shape and weight of a Deba fool you; they are nimble and precise. Choose the length of your Deba based on the size of fish you’ll be butchering most often.