The primary purpose of Filleting knives is to separate the flesh of the fish from the skin and bones. There are a wide variety of designs, each differing in terms of overall blade shape, thickness, grind, and also flexibility.
Each of these properties subtly effects the consistency and ease with which the blade can be run along the spine of the fish, just under the surface of the skin, in order to liberate the flesh. Consequently, it may be a good idea to buy the same style of filleting knife that you used during training.
Blade lengths typically vary from 150mm up to 280 mm.