A filleting knife is a knife with a flexible blade allows you to move easily between the flesh and bones of the fish – and the sharper the knife, the easier the job. Japanese created a special style of knives called Deba and Yanagiba, which are ideal for that kind of job. A standard fillet knife can have a blade of length 4, 6, 7.5 or 9 inches. A 6-inch blade is ideal for cutting panfish such as crappie, perch and bluegills. A small trout and bass are slightly longer than panfish hence a 7.5 blade would do the job, right for them. All fillet knives must be flexible. Fillet knife blades are made very thin, approximately 2.5–3.5 mm at the spine, so that they can still bend and flex and maintain an edge. … However, if the blade thickness is small, then the blade can be hard enough to maintain sharpness, and still allow flexibility for precise cuts.