A filleting knife gives good control and aids in filleting. It is a flexible member of the boning knife family that is used to fillet and prepare fish. Fillet knife blades are typically 15 to 28 cm long and this allows it to move easily along the backbone and under the skin of meat.
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swibo filleting knives
Victory Filleting Knife (diogenes)
"The knife I consider the best fillet knife ever made is The Diogenes 22cm. They are such a versatile knife and although some guys like myself use a limited range of knives and others have an arsenal, one thing is common a Dio as we call them can be found in every knife pouch of every guy I work with. Something that cannot be said for other knives." Konway from Fillet Fish Australia https://www.filletfish.com.au/
22CM FILLET Extra narrow