The reason that you're not supposed to use your regular knife to touch fish (or cut potatoes) is that a non stainless steel knife would tarnish. A fish knife i

The reason that you're not supposed to use your regular knife to touch fish (or cut potatoes) is that a non stainless steel knife would tarnish. A fish knife is silver plated and therefore safe to use. While my grandparents still had knives that weren't stainless steel, mine would be all safe and the rules now seem arbitrary.